OP, thank you for bringing this up.

I think we should be
(1) very careful about the package when we purchase the card
(2) use/unload the card as soon as possible and do not stock a lot in hand
(3) keep all the receipt for each GC

If you do face a fraud issue, besides calling the card issuer and reporting to police, please also give a report here. That will benefit other MSers, and help to stop this fraud. The more detail you can post, the more helpful I think.