Not having had to mess with one yet, what does the head of the idle screw look like? It may be possible to use a wire or piece of metal to hold it so it doesn't turn. Often bolts and screws on aircraft have holes in the head so a wire can be threaded through to keep it from loosening. Knurled knobs lend themselves to having spring steel edges pressed against them to keep them from turning. Screwdriver slots can also have spring steel pieces fashioned to keep them from turning.

-- Bryan