It's an excellent example of the fruitlessness of technical solutions to people problems. Some people are just destined to get scammed, and it isn't worth throwing away General Purpose Computing to try to help them. Be present in Grandma's life and she won't be desperate to trust the nice man on the phone just to have someone to talk to. If it weren't this it would be iTunes gift cards, or Your Vehicle's Extended Warranty, or any number of other avenues.

The actual stopping power here is that any grandma who uses a Linux desktop has a family member (or other contact) who helps with technical matters. They've been educated about internet & phone scams, and will immediately call their technical contact when anything is suspicious.