Is there a name for the red circle and slash in a No Smoking sign?

When my daughter asked me and my husband about a No Smoking sign, we explained that it means no cigarettes are allowed. (We had to explain cigarettes to her too.)

Unfortunately, she now thinks that the red circle and diagonal line are called a "cigarette", so she'll talk about the cigarette in the No Parking sign:

Is there a name we can teach her for the red circle and slash?

Solution 1:

No symbol.

The no symbol (also prohibition sign, no sign, circle-backslash symbol, nay, or universal no) is a circle with a diagonal line through it (running from top left to bottom right), surrounding a pictogram used to indicate something is not permitted.

Trivia: There is also a popular "forward slash" version:

Solution 2:

If I had to give a "stickier term" which also explained the meaning of the red circle symbol with a diagonal slash, I would say:

That's the Do not do sign.
Do not smoke; do not park here; do not use a mobile here¹; do not take photos ² etc.