The one used as a symbol of prohibtion, often on traffic signs? I remember hearing that there was a name for it and I do not recall what it was.
In Traffic Engineering we call them Prohibition Signs collectively. Like No Parking, No Left Turn and the like.
I call it the “Don’t” Sign.
It was a single word. It was a trivia question more than once on the radio.
The Manual on Uniform Traffic Control Devices (MUTCD)
Section 2B.19 Turn Prohibition Signs (R3-1 through R3-4, R3-18)
Standard:
Except as noted in the Option, where turns are prohibited, Turn Prohibition signs shall be installed.
Just say “no.”
I believe I’ve heard the word “interdit”, but I’m having a hard time finding any non-French sources for that online.
The “buster.”
Heh, my family calls it the “buster” as well, because of Ghostbusters.
We just call it the “circle slash.” Not too clever but everybody knows what we mean.
The Nomor.
The Nomor Garriceparra? 
I used to have a book of Sniglets (remember those from the “Not Necessarily the News” segment Rich Hall used to do?)
It called that thing a “Nomor”.
But then, it had lots of funny and apparently made up names for other things as well. To this day though I call it a nomor, often to the confusion of others who didn’t spend as much time as I did reading my “Sniglet a Day” desk calendar.
The Fuck You - first used for smokers I believe.