To piggy back a bit, I can block people from kbin, so I've thought about this a bit.
What if you are in an instance that has users with great memes. Someone csam person joins and you just block that person. Maybe you're a little proactive and go through comments and block any supporter that's supportive of the csam.
A month or two goes by, the memes are still good, but you talk with someone from another instance and it turns out you're on a csam instance. You had no idea because you blocked all the csam content.