Disabling perception on "killed" NPCs

What DomusLudus says.
Setting the sense enable(False) doesn’t work. At least not with a blueprint node.
It’s possible that it works by toggling it in Code… which I have no idea how to do

Did you try what I said? Calling RequestStimuliListenerUpdate after SetSenseEnabled?

Use ‘unpossess’ node in your character bp to unpossess the controller.
Also, have an is dead decorator on the top of your behavior tree that’s set to false.
Have the character set it to true on death to stop all node processing.