Recently I have discovered a way to keep Pandaria rare spawns dead until server reset. Basically all you need to do is have someone added to your bnet friends list that currently has a level 90 on another server. Engage the rare spawn you wish to keep despawned (Have not tried with any of the Zandalari rares) and then have your friend invite you to a group. You will not be transfered to their realm until you leave combat AKA kill the rare spawn. Stay grouped for about 10 minutes, You can not loot the kill if you wish to keep it despawned, however the body will be lootable up until the server reset.