generally to get rid of people who have dynamic ip's you just ban their range, then after a few weeks unban it so other people with the same range can connect again. rarely do troublemakers attack the same server twice in a long timespan, they usually move on after a week or so.
also, unfortunately since OA doesnt need a cd key of any sort so doesnt have an authorizing server, more malicious people can still get around ip bans with a proxy. even if it did have an authorizing server it wouldnt stop people from making a keygen
just keep that sort of stuff in mind when you ban people.