This should have no impact on your ability to ban him. Just kick his ass out, and keep kicking him out until he runs out of patience.
Forum motion will not take any action against you. In fact, I cannot picture them taking action against your for selectively picking and choosing who you want on your site. Ban him, and leave it at that.
In my experience as an Administrator of a forum (the most commonly targeted website on the internet for an attack) the most decisive and absolute action is always the best. I have even banned entire countries from TNO to enhance our security. I have so many automated procedures that users must hop through to become members that it scares away potential attackers because the effort is simply not worth it.
Ban him, and keep banning him until he gets tired.
03-11-2012
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