I wouldn't tell them you know anything. They could have someone else rent and then do the angry renter thing anyway.
I would just tell them it's booked or if you are suspicious, tell them you have another inquiry that you have to tend to first, then tell them that it's booked.
If they are already a trouble maker, just let them down as nicely as possible. Tell them they might want to try some other web-site. Mention one that's not used much in your area.
I hope this helps!
Hello from the offices of GuestChecker!
My name is Shane. I am one of the founding members of GuestChecker. There has been a lot of talk about our service on the HomeAway Forums. I would like to open the field and answer comments, questions, concerns about our service on our new forum here: http://community.homeaway.com/message/10427#10427
I look forward to helping a community that has help us so much over the last 12 months.
Talk with you soon!
Actually, I "would" tell them that they are not a good fit for your property based on the information on "Guest checker". The people that leave our homes either messy, damaged, or write a bad review for ransom.....need to know that we are on to them. Maybe that will curb some of these entitled folks and make them think twice before acting inappropriately again!