I remember an article claiming that Reddit had it relatively easy in dealing with disruptive groups compared to other social media system is that they have a ban evasion rule. So that when a Subreddit was banned and created a new sub they could be banned again before creating problems.
Is the main side-wide problem that these posts turn up in the search results? Otherwise they are only seen by people subscribing the moderated communities people subscribed to?
If yes, an option for (low volume) spammers could be to only exclude them from search results and otherwise have the moderators of the communities and their downvoters deal with them. As you already write many cases are grey areas, so maybe in such cases such more subtle mechanisms are enough.
As only removing from the search results is less disruptive one could moderate more posts this way.
Replace them with cat emojis?