Help with Ratings feature: how to limit spam?

Hi all, I run an online accommodation reservation builder and I am going to add ratings now. Until two weeks ago I was quite confident that I could avoid spammers by limiting rating privileges to guests who paid through the system to reserve. That will avoid all the fake reviews TripAdvisor gets. But now, after 10 days in Vietnam I am afraid that's not enough. What happens here is: - Hotel Manager reserves its own room for one night for a Ghost Guest. - Ghost Guest comes, sleeps and leaves (nobody comes actually of course) - Hotel Manager leaves a great review. Cost of the great review? A few U$, the commission paid to the online reservation system. That's a great deal as it WILL bring many customers.

I am really sure this happens because I stayed in hotels via Agoda.com which were "fantastic", "best breakfast ever", "best hotel in Asia" and BS like that. The hotels were fine, but the ratings unreal.

So, one idea I have is to give a weight to each rating, proportional to the number of nights, giving a particular low value to the ones for 1 night stays, which are probably 99% of fakes ones.

Do you have suggestions?

thank you in advance!

Luca