You need to do it with user accounts so the search engine team can remove fake clicks and ban the user. Users with a low amount of ratings won't have any say in the rating of the site, since they will have a low confidence.
They will first need to rate a few hundred sites (valid ratings, because if they don't have high validity with the rest of the sites in the database the confidence this is a valid user will be low) to give the spam site a high rating. So they will have already built a database of good ratings before they can give the last vote that they get paid for?
Also, you can manually remove a spam site that uses this tactic and all their money will have been wasted.