D.B.
They say that little ones get so accustomed to hearing "no" because they hear it so often that they start to tune it out entirely. So we started using the word "freeze" when its something dangerous or serious. We even practiced this when teaching my 4 yr old to ride his two wheeler. We kinda played a version of red light green light.....but instead using "freeze" when we needed him to stop because he was getting too far ahead. The concept being he would literally "freeze" in whatever position he was in...he thought it was hilarious to be "frozen" and it helped him to comply in really serious situations.
Hope this helps. It took very little time to instill this and we still use it to this day.