I dug into the data that the USPS provides for addresses and found a few interesting things that can be used as clues for finding spam.
Probably the most reliable piece of data is whether an address is categorized as a Commercial Mail Receiving Agent. These are addresses that are handled by a private mailbox operator registered with USPS, meaning they're authorized to open your mail on your behalf. If you see a GMB listing with a CMRA address, 99% of the time that's going to be a virtual office or UPS store.
USPS can also tell you if an address is classified as Residential or Commercial, which can be a quick filter for which listings need a closer look. They can also tell you whether an address is marked vacant, meaning mail is no longer delivered to that address.
I've had pretty good luck using this data to quickly track down spammy listings. I've found the easiest way to get the data is to go through a tool like SmartyStreets (they have a 1 at a time address lookup page which is free).
My pleasure! I've been looking harder at spam lately, and what can be done with data that's out there. I'd like to try using Moz link data next. Their domain spam score might be helpful as another signal to surface spammy listings. My thought is that people who "rank and rent" on GMB probably have a spammy link profile as they try to use tactics like PBNs to pump up the rank.
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