Why not leave them blocked?
Then they know and they will just move to their other account and keep screenshotting or posting. The above method was totally stealth. They almost never noticed. I would casually check back sometime after a few days and every single one stayed unfollowed. They never saw it. “Quietly” is the keyword. If I wanted them to know I’d have just blocked them. But that makes morons even angrier and they almost always just return in some other way with a new vendetta against you