If you want to control this kind of thing, require that users turn over something that can be tied to their real-life identity, like a cell phone #. Then verify that what they give you is real.
Then make them swear that they have not received or been offered any compensation.
If you later discover that they probably lied, you can sue them.
As for people logging in from countries where suing the person individually is not an option, one thing you can do is limit the visibility of their comments, perhaps by limiting them to others who are from the same country.
Yes, there are ways to get around this (proxies, VPNs, disposable phones, etc.) but it will raise the cost of doing business for those companies who do "shill reviews."