As I said before, I've tried every technique going but it hasn't prevented some of the repeat calls happening. I've been polite, I've been challenging, I've been rude, I've put people on hold, I've asked to speak to supervisors (and sometimes spoken to them) - nothing has worked so far. I won't do the whistle thing because I have more respect for ears than that.
I'm very happy to hear that some of you have been successful by being polite but (trust me on this) that technique doesn't work all the time.

I don't think they necessarily all work for thieves, either, btw - yes, the "something wrong with your computer" brigade do, but many others are just using out of date lists or inappropriate scripts.