Questions about reviews and ratings

Hi

I have set up a directory using Responsive Business Directory theme and everything seems satisfactory – except for the reviews and ratings.

One of my advertisers has complained about the fact that anyone can just give her a rating without having to be a previous customer of hers. They can just click on the stars and give her one star. Worse still, it would appear that they can just keep clicking on it and give her loads of one-star ratings. Is this correct? And if so, is there a way to stop people giving an advertiser multiple ratings?

Thanks.

Nick