Hi Jeremy,
Good questions, and unfortunately, the bad news is that review spam on Google is rampant, and unfortunately, does not appear to be well-policed. Technically, yes, Google could both remove spam reviews and penalize the listing that is engaging in them (either manually or algorithmically), but as Mike Blumenthal has recently been documenting, Google does not seem overly interested in devoting resources to catching or penalizing review spammers (definitely read: http://blumenthals.com/blog/2017/04/17/the-largest-review-spam-network-ever-or-who-is-shazedur-rahman-and-why-should-you-care/)
There seem to be thousands of fake reviews in the network Mike has been tracking which violate not only Google's guidelines but also, likely, FTC regulations. Unfortunately, this state of affairs with Google appearing to ignore massive review spam calls into question the trustworthiness of their review product and it's something I would hope to see them crack down on in future.
Google's situation provides good background as to the famous stringency of Yelp's review policies. Yelp is highly invested in ensuring that reviews on their platform are legitimate. This is why they remove 28% of the reviews they receive and why they have publicly shamed erring accounts from time to time.
So, for now, if you're trying to convince a business not to spam Google's review product, you are likely to have to use something other than the immediate threat of penalties as your argument. This might include:
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The possibility of a future Google crackdown at any time, with consequences that could be as mild as a soft penalty with loss of the fake reviews, to as severe as being banned with all of the traffic and revenue your GMB listing used to drive for your brand vanishing overnight.
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The possibility of consumers discovering spam on their own and causing permanent damage to the brand's reputation via Word-of-Mouth, social sharing, etc., as well as the obvious loss of the customer and his network of friends if they are disgusted enough.
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The possibility of FTC actions, lawsuits, etc. This should startle any business on the review spamming road: https://searchenginewatch.com/sew/news/2296366/fake-online-reviews-cost-19-companies-usd350-000
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The weakness of a marketing strategy that relies of faking success instead of actually achieving it. You can't use fake reviews as a benchmark of growth, gains, quality control or anything like that. You're just fooling yourself, instead of putting in the work to achieve a genuine reputation for excellence.
You may think of other discussion points, but these four should be enough to convince any legitimate business with even a small amount of concern for staying in business that these risky shortcuts are a hazard rather than an asset.
Hope this helps!