There is one, and only one, way to fix this---in my experience, at least (which is, unfortunately, considerable in this arena). That way is: dumping it onto another cake.
Forget about the stout for a while. Let it be. Next time you have a free few hours, brew up a relatively neutral (like a pale ale or blonde) low-gravity ale (~1.040-1.045). Pitch dry yeast like US05 or Nottingham, let it ferment out. When it's done, rack off to another vessel. Then, rack the stalled stout onto that new yeast cake. Then, watch the magic happen!