I've blocked, like, 5 HexBear users and then haven't seen a single shitty post or comment from that instance since. Either they are all banned from the other instances/communities, or it's really only 5 dickheads.
There are a lot of dickheads, but a lot fewer of them intentionally reach outside their instance to harass other people. The problem arises when the originating instance's mods/admins take no action to control their brigadiers.
I had to clean up a bunch of obvious bot spam from FoodPorn yesterday when I noticed it all after accidentally clicking into the community and leaving my all feed. I am now not sure if the report feature works; all the posts had tons of down votes but my report inbox has never had anything in it.
If other mods are relying on the report feature and it's broken, it would explain why even the active mods are slow to react to things.