Luckily I had warned my mother-in-law about these - she's safely on Ubuntu, but I keep everyone in the family aware of scams.
So she played along for at least 45 minutes, before telling them their mother would be ashamed of them and they should get a useful job like sweeping the streets!
This seems like the best approach, if you can spare the time - tie up so much of their time that it is uneconomic.
Speaking as an Australian that's had a few of these calls, it's kind of fun stringing them along the first few times and then after that you can just hand them off to the nearest grandmother or 13 year old and let them deal with it.
My nephew (13) thinks it's great fun to stretch these calls out as long as possible, but then he's into playing Skyrim & what not.
So she played along for at least 45 minutes, before telling them their mother would be ashamed of them and they should get a useful job like sweeping the streets!
This seems like the best approach, if you can spare the time - tie up so much of their time that it is uneconomic.