I had a co-worker who was getting harassed on her work line from a fake money collection organization. So I started calling them, and calling them, and then they discontinued their number.
They then started calling her again a few months later and it took even less time for them to shut down that number.
As far as I know she hasn't had another call since.
Oh, and if at all possible try to figure out where they are calling from and try to use the differences in culture to insult them. Like calling them shoe lickers or something. Just calling them regular english put downs aren't as effective.
Also you may want to try the "why not do something better with your life" talk, after all a lot of times these people are better educated than many of their peers and could make a significant difference in their community if they weren't intent on trying to scam those rich dumb people from that rich dumb country.