IMO it's best to just handle multiclienters on a case-by-case basis, as there's no solid way to do this without loving over people connecting from the same LAN.
You could block multiple connections from the same bl_id, as I don't see very many people sharing the same id, but most people with malicious intentions are going to have multiple IDs anyways, and blocking connections from the same ip would completely forget over multiple people playing from the same LAN