I had one user I banned by going into their profile and changing their password and username but not their email. (That way they couldn't sign up again with the same email address anyway).
People can always use a different email address and username/password if they REALLY want to get back on though.
Not sure if this did the trick but I haven't seen him since and they wrote me an email asking what's going on and if they could get back on the forum
(Unless he re-joined under a different UN/PW/Email)
I'm sure you knew all this though....sorry nothing "new" to add