There are several functions to clean up old credit card information. You can run any or all of them at any time, depending on what information you want to remove.
Note: This assumes you've entered credit card information in the designated fields for Guarantee Information or for Payments and Deposits. If you put credit cards in other places, such as Notes fields, then that information is not removed automatically.
The first two functions, removing swipe data and CVC (security) codes, are recommended for PCI compliance. These functions sweep all of these from the entire database, but leave the other card information intact.
If you're reluctant to do the first two functions for PCI compliance, then there are three functions that can be used to selectively remove the swipe and security information from only the "old" transactions. This will keep the information for current and future reservations (e.g. so the card can be re-used with the swipe data intact), but remove if from old reservations -- that is, from the transactions of reservations already checked out (or at least ending in the past). You can also remove it from Unbound or Point of Sale transactions that have no link to a customer (so there's no need to keep that information).
If you want to completely remove credit card information (including the card numbers, expiration dates, etc.), there are several functions that will do that. You can select which transactions or guarantee information is cleaned of card information. Note that none of these will remove it from the Guarantee information of future reservations (presumably you still need that), or from transactions of future or current reservations (since you may need to process a void, credit, or additional charge).
Finally, while it's not a strong security risk, you can clean out all expired card information. Expired cards won't be re-used by accident anyway, so this is simply a minimal clean-up operation in case you don't want to do any of the others. Keep in mind of course that this will still remove some "history", in terms of what card was used for past transactions.
Important: These functions can result in a large number of data changes, potentially causing delays if you're networked, etc., especially if you have never done these before or only do them on rare occasions. At the very least, close this dialog after each operation if it shows that more than 100 records were cleaned up, to allow things to be synchronized and log files saved.