typically they are banned from the site as well as their reviews or anyone who will try and post them.

Again, I am not a TER employee but know there are a few ways for websites and their administrators to tell.

The most common or easiest is when someone tries to post multiple reviews on one provider and its from the same IP address.  

One that always raises an eyebrow from me is when a review finally gets posted for a provider and the reviewer rates the provider very high and then when you click on the review, its a few sentences of broken english or sometimes mirrors some verbage that may even be in their ad.

Thats my two cents....hope this helps.