What transmitter do you have?

My transmitter has mechanical trims. I pull the throttle all the way back, and the trim all the way back. Then I adjust the pushrod so the carb is fully closed. When the trim is moved forward the carb opens slightly and I can use it to adjust the idle.

If you have a computer radio with digital trims, look for a "throttle cut" function that is typically assigned to a switch. This function could have a dozen different names depending on the brand and model of your transmitter.