You cannot change your username nor can you directly deactivate your account according to this: http://forums.asp.net/t/1933703.aspx
typical.
AndI tried the contact link at the bottom of the page takes you to a maze worthy of phone-tree envy.
I was only able to deactivate my account by posting a message in the "Feedback on this website" forum: http://forums.asp.net/188.aspx/1?Feedback+on+this+website and wait for a moderator to handle it.