I just shoot them off right there.... Ha-ha-ha! Just kidding...
Those parks I go to are NOT great for biking, so I rarely see someone. Well... My paint actually hears them coming from back (or front) WAY before I even realize that. I can tell from her move something is coming, so I usually stop and make her face them and let them pass. So I just don't believe ANYONE can come to her that close from behind - she's on 360 degree check constantly.
If bikers are coming towards us most of them stop and let me pass.
The general "rule of politeness" around is when the biker coming from behind to let the horse people know (s)he's coming so they can stop and let him pass on left.