There is no stopping them, one of their miscreants said to me- "if you buy their services the calls would stop", - arrogant eh! Well, I bought me a whistle, one like those used on the football game field, with the rotating ball inside, gives an extra loud shriek. When they called I'd give them a real loud BLAST as loud as I could.
That seemed to have stopped them so far.
Probably need to expand this as an anti scam campaign.