Accounts are not deleted, not even for obvious spammers, due to technical limitations of the system. If you don't want to use the forums anymore, log off and don't return. We'll miss you!
Usernames cannot be changed either. If you want to use a different username, then create the new username and stop using the old one.
There are many systems that your forum account hooks into: support forums, make blogs, core SVN, plugins SVN, theme SVN, meta SVN, and so on and so forth. It's a technical mess and we know it's annoying when you want to walk away, or regret the username cocopuff as not professional, but it's just what we've got right now.