Block users by post
When you click on a post, it redirects you to a page with the post's content and the post almost certainly has an ID of some kind. So what we should do for when there's a user who spams garbage posts is include a button on those pages that allows you to block the user by their post ID
Say BigUser has an account ID of 1. Say they've been making a lot of trash posts so their most recent trash post's ID is 4321971. When you click the button it would run a simple database transaction:
Take the post ID of 4321971 and check each user in the database to see if anything in their post history matches that ID. If it does, then retrieve OP's user ID and block them with it