Hello from Vancouver. I am struggling with this same issue and it is getting worse as Airbnb becomes more popular. I have seen probably every kind of attempt to over-crowd my house.
Two years ago I had a limit of 4 people, no children under 12 years. I accepted a booking for a family of four. Several weeks later, the guest informed me that they decided to bring their 10 year old daughter, too. (Yeah, right. You were going to leave her home alone when you went on vacation?) I said no, they understood my concern and I cancelled their reservation. This was two months before their arrival date! The next thing I knew, I lost my super-host status with Airbnb for cancelling! I called multiple times, wrote letters, and nope, they wouldn't budge. I didn't know that I had to ask the guest to cancel. Finally, a year later, Airbnb responded to me and said it was the guest's fault and I shouldn't be penalized. But I had to wait until the next cycle to get my badge back! It took 15 months to regain my status. I was livid.
Just this month I have had two violators. One guest showed up on my doorstep and said they had invited their adult daughter to join them on vacation. She would have exceeded my limit and there was nowhere for her to sleep. I said to him "where is she going to sleep?" He said, "I don't know." Nothing like being put on the spot while guests are literally standing on your doorstep. I let them in and made up a rarely used sofa bed for the extra guest. My lisiting does not include a fee for extra guests because I don't want to encourage this behaviour. So, I felt cheated. I was afraid of losing my status again if I turned them away. Or, they would have simply invited her into the house after I left.
My last guest was a young man who had good reviews. I was wary of him travelling alone and addressed the issue with him directly. He promised he was the only guest and would not, not, not, let anyone else stay here. Of course, there were 3 used beds when I came home. When questioned, he readily admitted that he invited two friends to stay with him for a few days. Again, no extra fee to me.
I am wondering this: can I specify that I will charge a penalty from my security deposit if I discover that this rule is broken? I am completely fed up with this problem and it happens quite often! Thanks for your thoughts.