Guest broke house rules then left early now asking for full refund

[Alexis24]

So I had a guest stay with us for one night over the holiday weekend. They booked for four guests but ended up throwing a party where at least 30 people were in attendance. (We found out later from a neighbor) Our house rules state no parties for good reason. Our well cannot handle large amounts of people and our water is almost non existent when it's constantly in use. Well it was non existent and we gave them several gallons of water to use in case of that happening. They ended up leaving around 11-12pm since they were out of waterband are now asking for a full refund. I don't believe they deserve a full refund due to breaking the house rules. Any advice on what to do?