How to tell family I can't watch their dog in my home agin?

So i've been dog sitting for a few year now, but i'm having my first rover boarding clients this weekend.

One of the dogs has turned out to be a bat fit for me. I have two dogs and when we all had a meet and greet at a local park it seems like everything would be fine. As soon as the dog got here though and his owner left and it was clear that he was going to be a problem.

He is very jealous over human attention and was growling my dogs and humping them when they came near us, to the point where my dogs just went outside and were scared to back inside for like a whole day. Besides being rude to my dogs and the other dog i'm watching this weekend he also has some other really rude behaviors. Our backyard has grass and sand, but for some reason he will only pee on the cement and when he poops he does it right on our doormat or right next to the door. I've tried to have him potty on his leash but he won't and he never goes potty on our walks.

I've had to spend this whole weekend with my guard up every single minute of the day to make sure this rude dog doesn't start or cause a fight with anyone. He is smaller then my dogs so I have to worry about him getting hurt if bothers them too much.

The owners talked to me when we met up the the park about future vacation dates. There is just no way this dog can come back here, what should I say to them? Should I be honest and tell them that their dog shouldn't be boarded in a home with other dogs or just make up some excuse as to why I won't be available?