Ya know, I did just that! and they still got in! 
But I have resolved the problem now by deleting the account and requiring a valid email address for them to sign up. Now I can block the email address.
The funny story behind this, is after I resorted to the email thing, they tried to sign in again using a fake email address couldn't get in and wrote me a letter apologizing and begging me to please let them post.
As I am a firm believer in second chances, I allowed them in, they have made 2 posts and gone away, guess it's not so much fun to have to be nice huh? LOL
Edited by Susan0722