i actually havent encountered this (though i saw one when i was driving in my car), i would just go as close to the curb as possible to force them to go on the outside towards traffic. no reason you should have to risk yourself because this dumb ******* is going the wrong way.

on the sidewalk (in places where most people wouldnt ride in the road) i have no problem with people riding on the left side, i even do it myself. but in the road noone should be riding on the wrong side....