If you remove the compass and look inside you will see a screw that is in-line with the throttle control. Tighten it just a bit and it will create friction on the control shaft.
There may be another device down on the cable housing in the engine compartment that looks like a stainless clip with a bolt in it. Tighten the bolt and it deforms the cable housing increasing friction of the cable.
I like the first method as it doesn't permanently deform the cable and possibly shorten its life.