Anyone who believes that the IRS or Microsoft would really cold-call people is overestimating either's organization's level of support by several orders of magnitude.

It would be fun to give those chumps some fake credit card numbers to pass around.

Here's a tool that generates random credit card numbers that pass mathematical verification, but are actually fake, for things like testing online shopping software.
http://www.mobilefish.com/services/credit_card_number_generator/credit_card_number_generator.php

If they just collect the numbers and don't charge them till later, you can poison their database of CC#s with fakes.