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!!!!