Basically, you can't. Neither will the administrators who can, (they only do so on rare, important circumstances). If there's a simple problem to why you don't want it anymore, just log out and don't sign back in.
No accounts are deleted. If you don't want to log in just don't. Simple as that. With a database of the forum composed of so many stats, posts, vms, blog entries, ip information, etc etc deleting an account brings a headache of errors and issues with the database. So, if you don't want to log in anymore, don't. Nothing will happen to your account. Also, try to search a bit. Not even a week ago this same question was asked and answered.