I'm afraid the only way to get rid of them is to buy a new computer when the hard drive gets full. It's one of those crazy Macintosh things you hear about sometimes.
EDIT: New information has come to light--you might not need a new computer after all. Find the files you created (by default they're in user > music > GarageBand). Click on the one you want to delete, then either drag it to the trash or hit the delete key. If you want to delete more than one, command-click on them and hit delete.