*To Me* "No" Means No generally. Especially from my *M*. And a Safe Word is set up to be just that, a Safe word. No is not a Safe Word, imo.
Perhaps make it clear to whomever one is playing, that the word "No" is not considered an acceptable Safe Word, and make sure another is Chosen beforehand?
As a few here have said on this Topic, No means Yes to them. I also Agree in a sense with Sir Russel in that it's important for the Dom/me, Top, or *M*, to be well aware of what is going on with the Submissive or Slave they are playing/sceneing with, to avoid possible serious problems later. Including a Submissive or Slave who won't be playing with you anymore. Of course Playing/Sceneing with PPL one Knows is the best way to avoid these things ime.
Respectfully~SidheWolf