My issue with this entire thing is actually pretty simple:
You're trying to prevent an action (casting a spell).
Blocks exist specifically to prevent actions.
But you don't want to use a block. THIS here is where the whole thing gets fishy for me.
Attacks don't prevent actions, they damage your opponent in some way. If you're trying to say that you're breaking down his connections to his magic, I'd buy it. But you're not. You're very clearly trying to do what chi-blockers do on the Avatar cartoons. That comes off as a block (with a specific duration). But you don't want to block.
Whenever I'm GMing and I have a player who wants to do something that there's a rule for, or pretty much a rule for, but wants to do it using different rules, I assume they're either trying to pull something over on me (like bypassing Toughness powers and the majority of the defensive capabilities of OW, in this case), or they just don't understand the rules.
I'm not sure which it is, so let me be clear. IF YOU'RE TRYING TO PREVENT AN ACTION, USE A BLOCK. Maybe a maneuver to place an aspect you can compel for effect. But blocks were designed for this type of action. It's how the system works.
I don't care what justification you're trying to use for mental stress, it's starting to seem like you really do know how powerful dealing mental stress is and want "the forums" to tell you it's okay and reasonable.
I'll say it one last time.
USE A BLOCK.
I might be wrong, seemed to me like we moved past this already. The issue now is whether a Spirit attack, directed via Fists, can deal mental stress. I think it can, Taran provided a fairly decent narrative of how that could work.
Ok, take the pressure point thing out of it a second. If I could use Spirit directed via a Fist roll rather than Discipline, I don't see why I couldn't have that Spirit attack be with the more...spiritual part of that element, inflicting mental stress and dealing consequences like Loss of Confidence, or Crippled Magic via bursts of fear, or despair or whatever.
Add the martial arts and pressure points back in, the pressure points are simply the places the practitioner as been taught to place these attacks, the martial arts is how the practitioner was taught to deliver such attacks. Still the same generalized thing, now taking on the flavor that was desired.
Is that workable, or does it still need more refinement, or complete reconstruction/rejection and why?