Have turrets apply passive AOE grievous wounds/healing reduction for enemies.
With sustain being so easy to get, champions like
who are the main abusers of this can literally chase you around your own tower, while tanking shots and your damage, and make it out with a kill and at least 10% health left. But not to leave out
and all the healing supports there to help them. They to can, while a bit less effective, pull this off relatively easy.
So by making turrets apply Grievous Wounds/ healing reduction (maybe like if only over 25% max health). And when the champion leaves turret range, the debuff is immediately remove, while reentering immediately applies (if they are over 25% max health).

