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Depending on what kind of Internet connection the trolls use, IP banning them could be pointless. The next time they connect to the Internet they may have a different IP.
If you ban by partial IP, then you could be excluding some potential valid users on the same network.
If you ban by username, they could just get a different email and re-register.
If you put them on an ignore list, they may eventually realize that no one is seeing their posts, etc...
It goes on and on... Is there any decent method to deal with trolls? Attrition and a decent user base is usually the only method. Sooner or later the trolls will lose interest or get irritated enough to go find an easier site to fester.