Normally, defeating it with this. [Hoisting up his Keyblade for a moment.] You just sort of... need to whack them with it.

[WHACK-A-HEARTLESS! I swear Disney World is missing out on a great new attraction.]

Abnormally, would probably be through magic. It's only a deterrent though. If you don't do it properly, the darkness around the heart will reform, and they'll come back.