You can cancel your card at any time. The last time I signed up for the 32 credit promo, I got the card, used it once and then canceled the card within 48hrs. All the credits posted no problem.

For this recent promo, I'm looking at a hard copy of the regs and there's nothing about a time constraint for your first purchase. The only mention is that balance transfers have to be completed within 90 days to get the extra credits.