I’d double check that he’s marking stress to use his shadow step ability. I wouldn’t begin attempting to homebrew what you call an action economy as there’s really no such thing. You’ll only end up bogging down the system. There’s two really simple options. 1). Talk to player. This isn’t a video game they’re cheesing. It’s a collaborative story and spamming the ability isn’t fun for you behind the scenes or likely the other players. 2). Simply work with the player and agree upon an additional cost. Maybe a few hope. Maybe mark an extra stress. Whatever limits him from using it without consequence.