Can one user unregister yourself?

hello guys Smile

my question is simple. one user without admin or moderator powers can unregister his account? i mean, exists this option in board?

i am asking this because i've locked all foruns in my board to just registered members can view/post. but someone that have a nick called "sniffer" have posted in my board but his status is guest. he don't have one registered account. how this guy do this?

in some tests that i do here, my conclusion is that he do a registration, made the post and then do a unregistration, if it be possible. :p

anyone can help me with this mistery?

thanks
[]'s