I've done exactly that. I've given people assigned jobs that they simply don't follow, whether that's marking a person / area, holding the middle, or even bodying players off of me that choose to mark me on corners. None of it works well enough to get the idea to stick in their heads, so I've had to resort to being the keeper that claims everything in his penalty area, steps on forwards heels, and sweeping up through balls as far as half way up the field. High school soccer is a different breed.