If a monster is proc'd and goes yellow (even once):
1) Reset the proc so player has to trigger again.
2) Monster loses proc and cannot be triggered again. or
3) Put the monster on a 30 sec timer until it auto depops

SE could also simply nerf the drops from monsters that link/aggro a player, or they could tie in Proc rate to how many monsters are currently on your party's hate list. So a group of 3 members attacking 4 monsters would have an extremely low (near impossible) proc rate versus a party of 3 fighting 3 monsters.

Simple and effective solutions. People would go out of their way to avoid pulling/linking/aggroing multiple monsters if they do not have enough people to keep claim.