I usually check out every now and then the "visitors" on the forums and have found that some come from certain ISPs and providers. I have been pretty successful in using Cloudflare's rules to block them. I do the blocking by ASN.
Those that I've blocked are coming from Amazon, HighWinds Cloud, China, and a few other places. I think I have about 8 ASN blocks which saw my unregistered "visitor" rate go down from a couple hundred to about 50 which seems to be pretty much real visitors rather than bots, scrapers, and other bad actors trying to find security holes.
I don't think Google cares about the member accounts on your forums. I just try not to let them use my bandwidth and server resources by doing the block on them before they even get in the front door. :idunno