I agree with Aeronut and the suggestion to solder up the hole in the throttle plate(if there is indeed one there to be soldered up).This is a cheap and easy fix compared to buying another carb.You should be able to kill the engine with your trim tabs if it is set up right.If your engine still runs with the throttle plate fully closed,you have an air leak some where.Your engines idle speed is controlled by your radio's trim tab setting,not the idle speed screw on the carb.This screw should be backed all the way out to allow the throttle to completetly close or remove it from the carb all together as it really has no purpose here anymore.The throttle arm return spring on the carb should also be deactivated so it is no longer spring loaded as this only adds extra load to the throttle servo and battery drain.Use side cutters and clip off the ends of the spring so they no longer engage anything.Do not remove the spring body though as it acts like a spacer for the throttle arm and keeps it from slopping all over the place.Hope this helps some.Happy flying.