Sorry but accounts aren't closed here. :(
If you want, you can delete your posts, or PM an upper staff member to delete your posts off threads that may possibly be locked. You could then remove all your profile information, and then if you want, set your password to anything random in your usercp, so that you won't remember it and then it'll be "closed" in a sense. Otherwise you could just leave PC as a whole, but yeah that's as much as can be done.