Firstly, I have two ways of interpreting a beginner tag. I may switch between both interpretations in this post to cover all bases, one interpretation is "questions in beginner physics", and another is similar to the one you ask "questions where the OP is in over his head"
Blast from the past
This reminds me a lot about all the discussions we had about popular-science. The major issue with such meta tags is that they're vague. So, allowing them means that each user applies the tag with their own definition of the tag. And it rapidly devolves into a "I-don't like this post" tag which each person uses differently.
For example, one user may consider the entire body of classical mechanics to be beginner level, one may not. The same goes for, say, newtonian mechanics. There are plenty of nontrivial problems that get asked in NM, too, and some may hide them with the tag while other tag users may not want that. Quoting Shog:
There's been an awful lot said here already, so I'm gonna cut to the
chase: popular-science is clearly a meta tag, in the same
way that homework is a meta tag - it can't stand alone when
describing a question.
That doesn't necessarily mean it's bad or should be removed though.
[popular-science], like [homework] has value in describing the sorts
of answers that can be found on a question... or should be provided
to a question.
BUT THAT ONLY WORKS IF FOLKS ARE CONSISTENT IN HOW THEY USE IT.
After years of support and many thousands of questions, we had to
burn [homework] on Stack Overflow because folks were using it
as a club to beat others over the head with. When folks are regularly
getting into fights over a tag, that tag has become a liability and
must be removed - regardless of how nice it might be in theory. And
before you say "it can't happen here", open your eyes - it already
is happening.
While I sympathize with Jim and others who wish to classify these
questions, Manishearth is dead right with regard to the ambiguities
and potential for abuse: these are the reasons why meta tags are
discouraged. If this tag is going to remain in use, three conditions
must be met:
(see that post for the conditions)
The reason the homework tag works well is that the tag is well-defined. I personally am uneasy about having a meta tag like that on the site, but after thinking about it, it seems to not be causing problems and would cause far bigger problems if removed anyway :P
Close them!
However, do not despair, there are better solutions to this specific problem. I think.
The questions are inevitably ill-informed and badly phrased.
[...]
I propose we need a tag that means the OP should go off and do some Googling around the area then come back and ask a more focussed question
Ill-formed and badly-phrased? Flag/vote for closure as Unclear What You Are Asking1. And comment to try to get the OP to do the Googling, if you're in the mood for that.
If (and only if) the user asks explicitly for a layman answer (and I bet there is a significant amount of overlap between such questions and the type you dub "beginner"), then we have popular-science, which has the following excerpt:
Questions in this tag must be answered in a manner that can be understood by a general audience with at most a basic understanding of physics, mathematics and related sciences. Apply it to questions where the asker clearly requests such answers.
Individual tag ignores
If one is concerned about the level of questions, it's pretty easy to ignore tags like newtonian-mechanics and company.
Come to think of it, we have many such tags. It may be good to have a meta community wiki post listing such tags in approximate increasing order of average expertise. Then, people can easily copy paste the tags they wish to ignore. (feel free to start this post if you wish)
Besides that (veering a bit off topic here into related ground), ignoring homework seems to get rid of a lot of the crap. Note that if you see a homework question that needs closing, vote to close and downvote. Closed posts that aren't downvoted still appear on the main page, and contribute to making our site look like a place to get homework help.
1. If you think that the closing system takes to long and have lost faith in it, let us know. Currently, the mods are holding back from the queue, but we increase our participation there as a temporary measure if there are problems. 3k+ users can help be part of the long term solutions by actively checking out the review queues and handling at least a couple questions from the close queue every day, if even 10 users do that we'd have a very fast system.