I reserve the word "stop!" for dangerous or disgusting things. I pair it with the sign for stop and use a firm tone. To that fact if dada says "what does mama say?" she slaps her hands together and says "nop!" which is her closest to stop at 14 months! I use it for things like running towards the street from the driveway, or reaching to pull a lamp down on her head, or going for the toilet again.
If/when I need to say no, I redirect instead most of the time. I get on her level and explain a lot to her too. "I know you want the cookies but we are done eating for today. Tomorrow we will have a cookie." Or "This is the last song I'm going to sign before night night time," as she signs more, more, more.
DD, on the other hand, says NO! all day long, to everything!!!!
At our Montessori school they say, "That is not available right now." I've tried this- but I find the distractions work better. "Where are the birds?"
I like the "Hands on the nose, etc" routine. I'm totally stealing that!!
I have also started using "STOP" for something dangerous or disgusting. We say "hand off" or "hot" for things that shouldn't be touched. I also say "Come out from there" and "Feet on the floor please." I totally loe "Think again," that makes me giggle and I think i might try it!