You probably don't have to cancel the cards, but it would be wise. Why? If someone managed to activate one of the cards and charge wildly on it, you'd get stuck with the hassle of proving that the charges were fraudulent.
Also, if you have a great deal of available credit, it can hamper your ability to get a loan. In the strange world of "credit scoring" -- a process that most lenders use to determine whether or not to issue you a loan -- this available credit can make you appear to be overextended, even if you've never used it.
As for how to cancel the card: Call the toll-free number on the back of the card, and send a letter to the company confirming your conversation with the customer service representative. It's a good idea to check your credit report within the year to make sure that the card has been canceled.