Locate a recent statement from the issuing bank. This will have your account number on it, which you will need to verify or enter upon contacting the bank. If you don't have a paper statement, check and see if online statements are available on your account. Without your account number, your bank will likely ask several security questions to verify your identity or require you to visit in person.
Call the 800 number listed for your bank on the statement and speak with a customer representative. An automated system may be in place for canceling debit cards, but its best to speak with a live person in case you have any questions or concerns regarding the account.
Tell the representative that you are canceling your card and state the reason. A lost or stolen card will require a different protocol than if you are simply closing the account for good.
Write down a confirmation number and the representative's name following their assurance that the card has been canceled. Keep this information in your bank records in case any problems should arise.
Visit a branch of the bank and cancel the card in person. Upon entering the bank, ask a manager to assist you, since some tellers will not be able to cancel your debit card.