I have repeatedly erased the email address of a member so that he cannot log in again. He keeps re-entering his email address. How is he able to re-enter his email address? The message I get on the screen when I remove his email address is that he will not be able to log in again.

I understand that, if I erase his name from the class list, all his pasts posts and profile will be erased as well. I don't want to do that. I just want to prevent him from logging in again.

He's probably just "joining" again. Enable the verify option before he can actually join.

Preferences, NEW CLASSMATE VERIFICATION

I think the message is more a generic statement that they will need to start over. And that's what he's doing.