@Traci1, there used to be a simple trick to get rid of that Extra Persons box entirely (i.e. enter a very large amount, hit Save, and when the page refreshed, the box would be gone). However, ABB seems to have caught on to the trick recently, and it no longer works.
All you can do is be very clear - in at least two highly visible places in the description - about the maximum number of guests permitted. Include it in your House Rules, which need to be very, very specific.
@Corinna8, that's a difficult one to handle. In principle, only the five 'official' guests should be allowed into the house. The remaining two would need to be directed to find other accommodation. Matter of fact, the official guests are already breaking the rules of the contract, especially if the maximum number is included in the House Rules as well. You could state that if anyone shows up that is not one of the official registered guests, AirBnB will be informed and the reservation will be cancelled immediately - for everyone.
You can reinforce this by reminding guests of this aspect of the House Rules during your communication prior to and/or after the booking has been made.
I'd also recommend that you require a hefty Security Deposit.