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