My usual response is to play along with them as long as possible, all the while asking questions like "What address to I send the giftcard to?", "Does it have to be a Green Dot?", "If I don't have that much right now can I make payments?", "I need your phone number so I can call you back when I check my accounts."

Then I use all the information (usually they're even nice enough to give me a website) to backtrace them and report the scam to appropriate authorities for the area they're in ...

Surprisingly they've stopped calling me