Just a little bit on the subspace part of this: Well in some peoples experience, depending upon the degree of disaccociation achieved, when one is really in sub space, they wont be able to respond coherently at all, so they wont be able to communicate anything about bringing anything to a stop or slowing down etc.

As Oz and I stated earlier (though perhaps I didnt explain it so well as he did) the best way is to puase and check on your submissive frequently and get verbal confirmation, something that is quite easy to do, even with loud music and other things going on in the background.