A subdual interference rule?

[Knight Of Pentacles]

I've seen incidents and experienced them first hand (regrettably I even did it once when I first started playing) where one party will subdual a PC but another one not involved in the PVP comes in and kidnaps or full damage kills the person subdued by the first party.   While this can be in-game, it is often very discourteous to the original party and the subdued player.  I would recommend a rule that would say for a second party to intervene they would either need permission from the first, have subdued the first, or have permission from a DM.