My radio has a feature called Throttle Cut, which is accessed by pushing a button on the radio. You set it up so that when you push the button, the carb closes completely. That way you don't have to mess with throttle trim.

You can either pinch the fuel line that is going to the carburetor, or with a rag (or something to keep you from getting burned) you can plug the exhaust on the muffler. Always be careful to stay away from the prop.