You paid your app fee. I'd wait and see what your decisions are. They generally accept a good number more then they actually hope will matriculate, so you're not really taking a seat away from anyone.
And if there's anyone that's really considered borderline, they'll be waitlisted. Then you turn down a seat, along with a bunch of other acceptees, the school will look to this waitlist to fill a seat.
The only time you're really holding things up is if you pay more than 1 initial seat deposit. That's when you're actually keeping someone else from filling it.
And in terms of *how* to do it, I'd say a letter would be the best way, with maybe a heads up via email. They might want something with your signature on it to withdraw an app.