I was wondering how to completely leave this community. Please guide me. I have gone through FAQ section but of no use.
What should I do ?
There are two ways to go about it:
Quit and keep your account: Just stop visiting the site. That's all. As far as Stack Exchange is concerned, it's fine if your account sits around unused. Leaving your account intact allows you to change your mind and come back to the site in the future.
On some other websites, a member who wants to take a break can ask for their account to be suspended to prevent them from accessing the site. But on this site, moderators generally will not grant such requests.
Quit and delete your account: You can find instructions for deleting your account in the help center, as other answers have mentioned. Account deletion is irreversible. Deleting your account means that your reputation and other accomplishments on the site, as well as your profile page, will be completely erased from public view. Your questions, answers, and comments will be anonymized - that is, your username and profile will no longer be associated with that content. You can come back and participate in the site in the future, but you will have to start from scratch with a separate account.
Again, the questions, answers, and comments you have contributed to the site do not get deleted along with your account. The legal basis for continuing to display your submitted content after your account is deleted is provided by Stack Exchange's terms of service, which you agreed to prior to submitting the content in the first place.
When you submit a request to delete your account following the directions in the help center, that request goes directly to the Stack Exchange community team. They may ask you some questions about why you want to leave the site, which I believe are for information only. I don't think they will refuse your request to delete your account regardless of why you want it deleted, but I am not part of that team and I can't authoritatively speak on their behalf.
The moderators on this site are not involved with the voluntary account deletion process in any way. We do not handle or even see account deletion requests. Although it is technically possible for a moderator to delete an account, we will not do so at the request of the account owner.
You can also look at the corresponding FAQ post on Meta Stack Exchange, but it doesn't really tell you anything useful beyond what you will find in the help center.
Exactly why do you want to delete the account, though? Don't get discouraged due to a few downvotes.
If you want to left only the Physics Stackexchange site then you should go here and enter some of the reasons or problem due to which you are quitting it. After submit check your inbox and confirm the submission and this is how your account will be deleted. But do say good bye to all who had helped you in physics difficult questions. If there is any problem then you can comment on my answer.
Account deletion is not helpful, with it you only give up rights and nothing get you back.
(Extension: it also gives up the possibility that once you get back and try to fix things what caused your wish to leave. It is much better to leave on a way while you still hold the possibility to once get back and try to fix things here.)
If you think enough is enough, it is a better idea to simply stop visiting the site any more - maybe once in the future you will need the privileges and the rep here.
You can't really delete all of your posts:
If you want to leave because some unhappy social situation, then don't worry. Simply stop posting, and nobody will remember you after some weeks (for old, active or particularly.... controversial accounts, it can be even a year, but not more). But you can come back any time if something would change your mind in the (maybe far) future.
If you want to leave because it is your only way to make some of your data unreachable, then note: most of the sensitive information is only available here for only a narrow circle old high-rep users (or mods). Consider a nickname change, or ask the help of the mods/CMs, they are sometimes cooperative, sometimes they want to be but can't (for example, also mods can't delete a single edit from a post history, they can only delete the whole post).
I think the only real reason to delete your account if you fight stackexchange addiction and it seems to be the only way out. But think first for these: