Kept one of them online for about an hour with, of all things an old Vista VM I had.
Let it go, and go - then came the sting for $$$ and I politely declined. Then the anger started, I enjoyed the threats for a minute or more - then calmly told them I had actually scammed them. He was confused, and I explained that it was a VM with a snapshot, and that as soon as he was off the phone the VM of Vista would be back the way it was before they called.
Then again, on another call - when asking there was a problem with "Microsoft Windows", I asked which one of my Linux or Mac's was causing the issue....
I enjoyed the threats of "we know where you live". The counter for that is, can you afford the airfare on your wage?