banned users can not register again

[steven s]

Only thing I can think of is moderate all new users on their first post.
It is a bit of a pain unless you have mods.

Copy registered usergroup and create registered2 usergroup.
Change registered so all posts are moderated.
Create a promotion so that after 1 post a person is moved into registered2.

This way the new person never gets past 0 posts.
Eventually the person will get tired, you hope.