Payback. I know it doesn’t prevent agent from spawning but does it stop the payback of monsters I have attacked on my turn?
You used the word "Payback" which actually refers to a specific skill described in the rulebook...but I believe you intended to ask if the stunned monster would get its counter attack. Is that the question?
If the monster's target is in LoS, or not in LoS but not in a shadow zone, the monster will spend its counter attack activation to remove the stun. In that case there would be no counter attack. If the target is out of LoS and in a shadow zone then the monster will not activate at all for counter attack, in which case it will not remove its stun until it activates in another player's turn or in the enemies phase.