I'm always torn between this and the advice that any response just validates for them that there's a real person behind the contact info and therefore it's definitely valuable to keep spamming.

I've had it work effectively in the past. I once had the guy waiting for 10 minutes while I found my 'password', which he was eager to have me read to him over the phone. You can imagine what it was. :) His response: "you bloody bastard, I <expletive> <expletive> mother .. "<CLICK>

My solution is just to play an extremely loud and annoying tone over the phone (such as a siren sound), not only do they hang up extremely quickly. They never call again.