Your language might be a little too tough. Is there anything you can add that will let them know that having more than the max guests will make it a less pleasant stay? Maybe make it less about the money and more about the experience they will have with the right number of people.
We had this happen on our third rental. Our listing states maximum of 5 people, but we didnt' spell it out in the rules that more than that were not allowed. The guests who stayed had friends over and were really loud (we live down the road, and could hear them from our house). I called them at midnight and told them to be quiet and asked if they had more than 5 people there. She said they would be quite and that their vistors had rooms at a local hotel and were going back that night. Of course they didn't leave and I noticed the visitor's car in the driveway the next day. When I asked her about it the next day, she said they had too much to drink and couldn't go back to their hotel and that "they all had jobs." (???). Truthfully, if she had just been honest about the drunk part when I called her at midnight, I would have been less angry.
After that I updated our listing to specifically say that we only allow up to 5 people unless other arrangements are made, and that it isn't a place suitable for large groups. When people ask if more than 5 can stay, I talk about the septic system and how I don't think it can handle that many people, and in general people stop asking at that point. We also don't provide extra blankets or pillows at the cottage, but let them know that if they need more, they can call us and we will provide them. I also changed the terms to be $100 per person after 5, so if I can show that they had more than 5 people, I can charge them extra.
I also think it helps to meet people personally when they arrive, and it definitely helps to live nearby so they know that you know what is going on, though I know that isn't always possible.
Your place looks lovely and like a great place to stay!