When you ban their ip through cpanel it bans it for the entire site. Unfortunatly if they are comin back that means they are using dynamic ip's, which means they will get a different ip each time they restart their computer. As long as they arn't spamming your forum with post I wouldn't even worry about it too much.

If you can trace it back to a company or organization who's doing it you can ask them to stop, and you might have a case to sue them for harassment if they don't