Free relays will still get mass adoption. And they will have spam. Even twitter cant solve the spam problem. You get what you pay for.
Discussion
The real solution is AI. Make people subscribe to someone's (AI assisted) mute list. Spam problem solved.
For communities, besides the AI mutelist, and besides spam, admins should be able to tell the clients what specific messages should be minimized/hidden; think of it as a temporary per-post mutelist.