That's really sad that people do this sort of thing. Some may think this is a game to them but it's not a game! I tend to agree with Tbone suggestion of offering the discount on 2nd visit after the review has been posted. Realize this kind of defeats the benefit for clients initially but at least the provider would get her review.
Personally, I tend to steer away from review specials. Some have worked ok but then some become too much drama. If the hourly rate is too high the discount on 2nd visit idea may not be of much help to the provider.
Your hourly rate and activities at that rate are everything in this business. I would appreciate the occasional PM offering a review special but don't want to be bombarded... Lol