You use a condition.

1) Create a new condition and name it. It must appear at the bottom of the condition list so that it has priority over all other conditions. Select which switch you want to activate it.

2) Select the condition and then go into the AFR (D/R) section of the Model Menu. There, select the throttle channel and set the GRP/SNG setting to "SINGL". Then set Rate A and Rate B to +0.0. Set the offset to + or - 100, or more, depending upon which direction move the throttle to the lowest position.

Now, when you activate the condition, the throttle will be locked to the lowest setting, which would be "off".