Whether or not you have a strategy to leach, conditions very often dictate that many pilots will use the same thermal. I don't think it would be appropriate to penalise the pilots who are later to arrive at the thermal. Anyway, I wonder whether we are addressing a real safety problem. I have had a midair and a few close encounters without either following or being followed.. I like the suggestion of asking for discussion of any close encounters at briefings/pilots meetings. I think it might encourage good airmanship.