That is rude, they gave you an opportunity to work for them. Don't be ungrateful. They may have cut your hours but you don't have to stab them in the back. Just tell them you've decided to terminate your employment with ___ company as of ____ date. Thank you for giving me this opportunity, but I must move on. Sincerely, your name and sign it.
If you're working in retail or fast food then that is going to happen. If you get another retail / fast food job, you can expect the same thing to happen. You don't always have to come in when you're not scheduled to. It's not harassing people, if they need people they have to call who ever is available. They have to cut hours if they're not busy, you won't always have the same amount of hours every week.