@Sean1066 When the guest booked for one but said we and you questioned him, did he then add a higher guest count to the reservation? If not, that was the first error. "5 or 6" isn't okay- the reservation needs to be for exactly how many people are coming- any more show up and you don't allow them on the property.
You need to message this guest and tell him the party is over, everyone needs to leave NOW. When a host sees something amiss, it's important to act immediately, not just wait to see what happens.
If the guest doesn't respond to the message, round up some buddies for back-up and go kick them all out.
Airbnb can't enforce your house rules for you- that is up to the host. It's your property, it's up to you to make sure you are in charge of what goes on there.
And once this little nightmare is over, remove the wording "designed for entertaining" from your listing description. And be aware that no one who is being honest books a 3 bedroom house that sleeps 7 for one person, even if they don't out themselves by saying "we".