Guests violated our “no pets” policy, What should I do?

[David-and-Annie0]

Hi all, need some advice: we are new to this and our second set of guests is there now. We have a “no pets” policy.

This guest messaged me a few days ago asking if they can bring their two dogs—one little and one boxer size. I said that although we love dogs, the answer is no. One of our own dogs tends to pee inside if she’s smelled that other dogs have been there. I explained this to him. He said he understood. 

This morning on our surveillance cameras, I see a little dog walking out the front door and coming back. I feel surprised and disappointed. 
Now what to do? I am thinking of messaging him, saying I know he brought a dog—at least one—and that I can either ask him to leave for violating this rule, or he can send me a pet fee of $150. 
Doubly frustrating is that these guests are renting our place at a very low rate (before I understood that “smart pricing” was not so smart!)—but that’s a different story. 

[Jesse-And-Quyen0]

We are facing the same thing and we feel very disappointed by this. We don't know what to do either. Fine this family for unauthorized pet or cancelation. Why?!