That is exactly what your problem is. That spring on your throttle bar should be tight at neutral and should push the throttle closed at neutral, if there is any slop in that spring you are not set up properly. If you give your radio a little brake trim this should move the throttle arm in the direction you want, if it starts to engage your brakes when you do that you may have to adjust your brake collars so they are not engaged when your at neutral and the throttle is being fully closed. If this fixes your issue you should set your brake trim back to 0 and make the adjustments from the subtrim setting in your radio.