How does one know if you are being serial down voted? How do you reverse it and stop them from voting on your stuff in the future?
Serial voting is well addressed in the Meta Stack Exchange post What is serial voting and how does it affect me?, and the short of it is that in essentially all the cases that matter, serial voting (which usually means someone going through your profile and either upvoting everything or downvoting a bunch of posts) gets automatically detected and reversed within 24 hours. The precise details of the algorithm are not made widely available to avoid people gaming the system, but the thresholds are generally enough that when serial downvotes really begin to bite the script will kick in.
One common point of confusion for new users is when a single post accumulates a lot of downvotes in a short expanse of time. Here it's important to note that users can only vote on a post once, so by definition this behaviour is not serial downvoting - it is just a bunch of different users who disagree with the post.
Serial downvoting usually looks like a series of votes on different posts (with few posts receiving more than one vote), often with those posts having been inactive for some time. If you do see behaviour like that, the thing to do is to wait 24 hours for the script to kick in. If it doesn't, then see What should you do if you're serial downvoted & it isn't automatically reversed within 24 hours? - in essence, that probably means that the downvotes originated from different users.
If you still think that there's a suspicious pattern you can flag one of the posts with a custom moderator flag explaining the situation. There is probably not much that a site moderator can do to help, since by design they cannot see voting histories, but if it does look suspicious they will pass this along to a Stack Exchange team member who can look and see if there is behaviour that requires corrective action. If you do raise such a flag, be very attentive to any feedback you get, and please be receptive to such information and apply it in the future; do not abuse the moderator flag privilege as it does put an undue strain on the site.
Another thing you said deserves mention:
How do you reverse it and stop them from voting on your stuff in the future?
The point of voting is to deliver a clear signal about the quality of each post to prospective visitors and to provide a way for quality content to be more easily visible and accessible; this is obviously an integral part of the model. As such, there will never be a facility for you to block any specific user, named or not, from voting on your posts. (This is distinct from voting irregularities, like serial voting and sockpuppetting, which are abuses of the voting system and for which the system already has comprehensive defences in place.)
That said, if you're getting significant amount of downvotes, what it probably means is that people's opinion of the post aligns with the tooltip on the downvote button:
This answer is not useful or
This question does not show any research effort; it is unclear or not useful. Downvotes on a post (particularly multiple downvotes) should be a flag for you to look carefully and critically at their quality.
Make sure that your grammar, spelling, punctuation and syntax is top-notch. The goal here is to build a repository of quality content that will last a long time; poor language shws & if posts r bdly spellled or the bad grammar then th pst becoms much h4rder t read & mch < useful 4 de futur visitors. On the spectrum between txtspeak and textbook language we are shooting for maybe just below textbook level, and the care and polish you put into your posts will directly impact their perceived quality.
Double-check that your content is very much correct, and be open and receptive to any comments that claim otherwise. Scour your references to make sure that you've represented them correctly, and make sure that you've represented them fully, provide those references in full, and then go scour the web some more to find better ones. Think critically about your post and look for ways you could be wrong, or alternative angles that cast doubt on your approach; if you find those then it can help to mention them as pitfalls to be aware of (without necessarily needing to delete your post). Recognize when you're out of your depth and be open to the possibility that you might actually not know the answer to the question.
Make sure that you are actually addressing the question as posed, and that you're not going off on a tangent or providing an unrelated rant. This means you need to carefully and critically read the question and identify what is and is not being asked; address the former and delete any content that addresses the latter.
Make sure that you're asking a properly self-contained, cohesive, well-researched, well-posed question, that the prospective answers will be of a suitable length and require a reasonable level of effort, and that you've provided all the information necessary for answerers to address your question.
And finally, to reiterate, this and similar questions can be answered by an appropriate search on Meta Stack Exchange. I would also strongly encourage you to do a systematic readthrough of the FAQ for Stack Exchange Sites, which contains clear explanation of all of the essentials of the dynamics on the network.
Jen, I've seen that your posts are attracting a lot of downvotes, and as dmckee mentioned it is not due to a serial downvoter, so chances are that the quality of your questions are not upto the mark,
I see that most of your questions are about blackholes and other pop-science stuff, It is great that you want to know about these topics, for which I would say that you are doing a wonderful job trying to understand science (considering most of today's teens), but there is a systematic way to do it. Any good answer to these questions will involve concepts of physics and math that I doubt you would have come across, In order to have a deep understand of physics you need to start with the basics and work your way towards the top. There is no shortcut.
So, I'd say you start out with basics and start asking questions about concepts you find hard to grasp, I bet nobody would downvote these
And please remember that this is not to discourage you, this is intended to motivate you in the right path
You should realize that receiving a lot of down votes does not mean that there is a "serial down voter" (in the sense that the down votes are there because the post is by you). It could also be that your posts are just of low quality, like this one: Spelling errors, no capitalization at the start of sentences, the word "perpetuate" dangling at the end with unclear meaning.
Actually this site is not appropriate for the enthusiastic amateur/teenager, i.e. somebody who has not sat through a course relevant to the subject under question. Discussing quantum mechanics at the hand waving level is not the purpose of this site.
I have answered some of your questions, because I am sympathetic to the enthusiasm displayed , and it is good to encourage open questions and the interest of the young in physics, but you have to realize that most trained physicists get impatient with naive questions. So you should shrug but try to learn from the negative votes, particularly if there is a comment explaining the -1 . It is all in the process of growing up in physics.