If someone leaves a Mythic + while the timer is still above the +2 mark (~5-6 mins for most?), have a popup to the person who used the keystone:
"Would you like to teleport your group outside of the instance and reset your keystone? A 30 minute cooldown will be placed to your keystone."
for the person who left:
"If you leave this group, you will be locked out of Keystones for this dungeon for 24 hours. Would you like to leave this group?"
If a group kicks someone from a Mythic +:
"Doing this will forfeit any bonus loot chests using this keystone, and will cap any keystone upgrades to +1"
What do you guys think since Blizzard can't come up with something?