How effective is are rating websites antispam?

(Note: I won't name any website here, as I have no time to deal with people suing me, but it pretty much applies to any rating based website).

Just wondering as I am traveling in Vietnam and I had the following conversation with an hotel owner (HO):

HO: I want to create an hotel reservation website as my friend makes 30.000 U$ a month with hers. ME: cool, how good are you with computers? HO: I'm very good. I can change IP ME: what do you change IP for? HO: I can leave good ratings about my hotel. ME: ???? HO: Yes, I do this a lot.

I always wondered how they dealt with it. It seems it's not that easy.