I put a note on my map indicating that runners must stay on the road surface, or may use the sidewalk, whichever is the case, if there is a situation like that. I have one (that will now become 5, since another set of courses will be using the corner) where the sidewalk follows the curb, but there is a shortcut across dirt. On my maps, I always detail that corner, and indicate that cones and a monitor must be used at that location. Not up to me to monitor, but if a runner knows he cut the corner, his time is not accurate (if he looks at the map).f
Similar to what I just read a few days ago - "You can give someone the facts, but you can't make them accept the facts" - all we can do is point out on the map the way the course was measured.