Why not stipulate that you need to have been in that, say county, to make a review?
Yes I'm aware of GPS spoofing. It raises the bar for doing extortion attacks like that. Amrit the Dalit scammer who lives in Delhi has NO business in anything in Effingham IL... Unless they've been there.
It would make this sort of stuff harder to do. Not impossible.
I've posted several reviews in my home town, Google knows it's my home town from their maps data, that's a strong trust signal that Google could find a way to use to provide improved reviews. Google can probably tell if reviewers logged on to the restaurants wi-fi, that's a strong trust signal too -- Google could expose a trust status and still hide that they're essentially tracking our every move.
Again, they could be faked, with things like GPS spoofing, and using Wigle to determine local wifi there to fake that. But again, each step there significantly raises the bar of fraudulent activity.