How to manage when you check in your guests and after some time you realize that have invited more people to the house?

Hello I'm Anna from Malaga, Spain. Let's say you check in your maximum number of guests, in my case it's 6 people. 

But then you realize that they have invited friends or relatives to spend the day (we allow it if not too much extra visitors) but also some of them have the intention to stay overnight, sleeping either on a couch or 2 persons on a 1x bed. How do react in such cases? 

We have had that situation many times, even if it's literally written in our house rules sheet that we don't allow anyone from outside come to sleep if the sum up more than 6 people. Especially with Spaniards, because sometimes we get bookings from people who live in Málaga city but want to spend a weekend on a rural house with a good BBQ and a swimming pool. As we (landlord and wife) live in a separate apartment within the same yard, we can pretty much easily keep access control and identify when we have non checked guests. Problem is always the same, you let them know that they cannot stay overnight, they seem to accept the conditions and promise they'll leave before 11 PM, our deadline. But then either everyone goes to sleep or just lock themselves in and won't open the door as we ring the bell. And of course you cannot get in with violence and kick them out. 

Please share your thoughts about that or your experiences, both are welcomed. 

Regards, 

Anna