I had an idea, which I aired on a previous thread, that if everyone recorded these conversations,( or said they were recording them if your phone cannot do this), and told the callers in best official voice that the recording would be subjected to voice recognition software and included in a database for future reference, they might be persuaded to believe that they would be recognised and their calls blocked.
It does need a lot of similar responses, though, to get "urban myth" status and reach the perpetrators.
Perhaps it is even possible to really do it, but I can imagine them squawking about their privacy being invaded (tough)