The built-in list of holidays can't be edited, but individual holidays can be marked as Disabled. Organizations in Canada will want to disable the American Thanksgiving holiday, and Americans will want to do the same for the Canadian Thanksgiving (unless you're one of the lucky groups that gets both holidays off.)
Click on any holiday to edit its properties. Check Disabled to have it no longer appear as a holiday for greetings, job schedules, etc. Or right-click on the holiday name and choose Disabled from the pop-up menu. This doesn't delete the holiday, and it can be re-enabled at any point in the future.