Not much anyone can do for you. Your problem is with the lady who sold you stolen gift cards, not with Carnival. I agree that Carnival should have informed you the gift cards were stolen, but it's possible they were reported stolen after you got them.
There are alot of unscrupulous people out there and like your mother told you; if it sounds too good to be true, it usually is. It's always best to book through a reputable agent and pay what you know is the going rate for the cruise. Anytime you're getting a huge discount or getting something that sounds too good to be true, talk to your agent - they will keep you out of trouble.
In this case, you're out of luck and you're out the money you spent for stolen gift cards.