Not necessarily true. If you file a complaint with the FOS they charge the company in question £500 to investigate them and they'll soon bog off. I once had a similar situation, reported them after weeks of harassment and voicemails from a fake collector saying they'd visit my property. They never did and the company apologised, removed my number and never contacted me again. It does work, you just need to take the correct course of action.
As I said above, I've been doing this a long time. Making threats to an agent in a contact centre, a pure threat will do little if anything. Having a case and raising it through channels is different but even still credit and debt agencies have a job to do and the big ones will do it within TCF guidelines or will demonstrate they have been. Simply making a threat will scare no one, to my point.