Yes. you can use them. The credit card is only a means to earn miles. Every month the miles earned on a credit card is transferred to Qatar Airways privilage club, after which the miles on credit cards become zero. Starts accruing during the month again and transfer on the month end.

You can also earn Qmiles on other offers such as hiring rent a cars, buying tickets etc. This have no bearing on the cancellation of card. Card is only one means of earning the miles.

Hope I have clarified,