In the Adversaries section it does clearly state that it is at the GM's discretion weather an adversary is incapacitated or killed when its WT is exceeded.
Considering how easy and ubiquitous it is to apply stun damage with this system I am going to say that the only way for a player to ensure that they don't accidentally kill the target is by doing stun/strain damage.
If you want to incapacitate an NPC (especially a Rival or Minion) you'll need to specifically do Strain to them to cap off their WT.