It's based on dynamic count. For now it's a very simple how many users per post there are and each instance can set their own threshold for it.
It's not about few users, it's about a tons of users and no activity. If you have 5000 users and 3 posts, it's likely those are all spam accounts. This is what we're checking for right now.
This is not a manual process currently, but I'm planning to add the possibility to whitelist and blacklist instances manually in the future.