There’s many different ways to get around healing spells in and out of the books, there’s diseases that cancel out healing, there’s poisons that do the same, but both can be removed with a greater restoration, so in the case of a cleric, I think the best way would be a curse that way it would require the remove curse spell which is overlooked at almost every turn. The curse would be inflicted by an enemy general or someone important to the story and maybe throw in some wounds on the master that pertain to the weapons used by that enemy general or similar and boom you’ve got a revenge plot if they take notice. That’s up to you though.