M.R.
Telling him what to do instead of what to stop doing may be helpful. Show him that he needs to be one arms length away from other kids faces all the time. Practice this with him, make it the rule and avoid "don't" statements. give him "do" statements instead, and remind him of the rules before he has interactions with other kids. Give him a plan, decide before hand what the reward is if he suceeeds, make it something simple and free. I would tell him that it is never OK for him to touch the other children's ears, and that the concequence for doing that will be to go home right away, then do it no matter what.
M.