Like mentioned earlier...Say something to them about it. I hve seen it approached in a couple different ways.
Once I saw a young teenager scold the shit out of a full grown man with his son. The kid rode by and told him he needed to get the fuck out of the way, and as he rode off, called him a stupid fucker.
Probably not the best way to handle the situation, being as the guys young child was with him, but i assume it worked. I didnt see him again.