Yes you could of asked them all to leave, the guest broke the house rules, contract null and void.
You don't have to accept 'two people have gone so it's ok now' or any such nonsence, if you pre-warned them (and in this case twice) the guest has no come back, and if you have security in the building to confirm it, airbnb has to support you.
That said in future if you get an inkling that a guest wants to try this, be clear and tell them the building has security and there is a fine for extra's, which you will pass on to them, as well as kicking them out. Either guests will take the hint and stick to 6, or book elsewhere, or they they pay up (last resort)
If you have a deposit on your listing, start a claim with airbnb, show the communication pre-warning the guest about numbers, and the fine from security. They should give you a payout (should, not 100% guarenteed if the fine wasn't prewarned in house rules). It might take a while though.
Good Luck.