"Look, I know you're a criminal trying to scam me. You're not getting any money from me, ever. If this is a real debt, sue me, so that the court will make you obey the law, but it's no, its' a scam, and you're a criminal, so you don't dare do that."
Make it clear they're wasting their time, and they'll go elsewhere. The scam is a business, and lost time isn't profitable.
Aside from that, this is the outlet to all your frustrations. When you're dealing with a criminal to begin with, you can say anything, be as abusive as you want, and there's nothing they can do about it. If you threatened to firebomb their office, they wouldn't dare report you (and the NSA doesn't share).
Had a bad day? Scream obscenities in to the phone. Didn't get laid last night? Tell them you're masturbating while you talk to them. Relatives giving you a bad time? Question their mother's mating habits. Take out every frustration you have on the criminal. I will personally send you real money if you post a recording of you making the scammer cry.