OK, first off, let me state that one should never pull a knife just to "scare someone off".
When a knife is pulled, it is a last resort your-gonna-get-killed or you are in serious fear for your life type of thing. Pull your knife just to scare someone away may get you into seriously deep trouble with the justice system or worse yet, you may get your ass handed to you by the person you just pissed off by trying to intimidate him/her.
Ok, School's out.
Now, to answer your question, yes, I have pulled my knife on a would be mugger before and yes he did run away.
I'm damned lucky he did not have a gun or more skill, or even more resolve than me.
My ass may well have been grass that night.
I made the serious mistake of not leaving myself an out one night(about 9:00pm) when I had to use the pay phone in front of the public library. Basically, downtown was all but deserted, the library closed, the phone was located near the entrance and I had walls on three sides of me.
STOOPID! I was talking on the phone when I saw in the glass on the library's front window this guy size me up and start a rush at me.
I dropped the phone, turned around and flicked open my Military.
He saw this and promptly turned and hauled ass jumping a bench in the process of putting distance between us.
I cleared out right away too, as I figured he would flag down the first cop he saw and say I attacked him with a knife.
I didn't pull my knife to scare him, I had every intention of cutting him as badly as was needed for me to escape whatever it was he had in mind for me.
NEVER EVER PULL A KNIFE JUST TO SCARE SOMEONE! It'll be yer ass that goes to jail when they run to a cop and say you pulled on them.
Then there will be all that fun with lawyers and court dates and bail money and possible civil suits from the "victim". Quite a price to pay if one could have just as easily walked away. Knives are possible lethal weapons and pulling one should be only if an innocent life may be lost by not pulling it.