Recently, I hosted a dog who didn't end up being a good fit. The M&G was fine, and he was a perfectly sweet dog who got along well with my dogs, but the owners described him as being generally quiet, maybe barking once or twice when someone was at the door. While he was with us, he barked almost constantly, and woke us up at 3:30 every morning and didn't let us go back to sleep. He got particularly attached to my partner, and wouldn't even let my him nap - he started barking as soon as he was out of sight. For us, because we are in an apartment complex and share a wall with another unit, this was not a good fit. (That, and the not being allowed to sleep while he was here... after 4 days we were dead tired!) When his owners came back and asked how it went, I was honest with them. I said he was a very sweet dog, he got along well with our dogs, etc, but that he barked frequently while they were away and that wasn't a good match for our living situation. I still sent him home with goodies and thanked them for their business, and they thanked me for the feedback and for taking good care of him.
Being up front does a couple things - it gives them new information regarding how their dog acts when they aren't present. If you don't board your dog often, you aren't going to be aware of some of their quirks that come out when you aren't around. Second, if they need a sitter in the future, they go into that knowing more about the kind of sitter and environment that's right for them. Not only will they not request a booking with me, but they'll know he's not a good match for apartments or other places with close neighbors.
Of course this doesn't really work as well if the owners are the issue - but hopefully that's a rare case. In those instances, I'd use the Rover responses if they request to book again.