What have you communicated to the guest, @Christopher187? I would send a message in which you state that you are aware of the third guest on the couch and remind that there is an extra charge. Be prepared to be able to prove your observation of the third guest. Then use the request/send money option on the reservation to the guest. If the guest denies that there was a third person use the proof to seek a claim through Air BNB.
Meanwhile, provide a thumbs down review of the guest for not following the rules. You can count on a negative review from the guest. You can respond that the guest failed to follow rules.
What a pain, right? Just remember all the lovely guests you have had and move on. It is a business afterall 😛