I have this theoretical ban method that as far as i know has not been implemented anywhere and would require some programming.
Here is how it would work:
The banned user would see a slightly different board than the rest of the users, The banned user's version of your board would include his posts, but the normal version of the board, what everyone else sees, would not included them. To this unwanted participant it would appear that he is being ignored entirely, he wouldn't need to find another IP to post from, as it would appear that he is able to post. In theory he would eventually give up.

Some dickery with post numbers would be required for it to be entirely effective.

Just out of interest, whats the board?