I set 'qa_options' >> 'permit_select_a' to = 0 ( think it was 40 or 20 originally, as I had it set to [Mods & Admins] ). I never revisited the permissions page in the admin panel, for the reason that you mentioned. Still, Admins and Mods are free to select and deselect the best answer for any question. So, I returned to the database to double-check what was going on, and 'permit_select_a' was still set to zero (0). I don't believe that this issue falls under the instability problem that you previously (understandably) expected in your answer, but it defiantly seems strange.
I should also mention that, since I also do not want to implement comments whatsoever, I set the Admin>>Permissions>>"Adding comments" set to 'Registered users with enough points' and then set the newly added option that appears when that was selected, called: Users must have: [_____] points, to some number like "999999", IOW all but impossible to realistically reach! But my admin, mod and S.Admin can still add comments as they please. I was pointed to this dropdown option menu as a way of 'turning off' comments altogether, which I assumed, from the options presented, that I had effectively done, but I seem to be missing something...
EDIT: Just to be clear I changed the comments permissions as described in the second paragraph several days BEFORE I changed the database option 'permit_select_a' to zero (0) that you recommended for the best answer issue. Like I said I've not went back to the permissions page since then (i.e. the paragraphs are not ordered chronologically in time!)