How to tell someone that you dont want to watch thier dog again?

Hi, my wife and I are debating how we handle a possible future situation. We are currently boarding a dog that is not aggressive or anything like that, but he is not neutered and they have no plans to neuter him in the future. He has jumped a fence. But that is not the biggest issue. The biggest issue is that he is simply relentless in that he is constantly sniffing the other dogs. There have been very few instances where he will relax and just be. It has gotten to the point that he annoys the other dogs, they want to play and he just wants to sniff them.....all.the.time.

We have agreed that this will be the last time we will sit this dog, but we are not in agreement in how to approach this with the clients. I think we should be upfront and tell them that we will not sit him in the future. My wife thinks we should just let them pick the dog up and deal with it if it ever comes up. They didn't make it sound like the board him very often.