Wikipedia calls it the "No Symbol" [0]. ISO 7010 calls it the "prohibition sign" [1]. In Unicode, it's available as a combining character with the practical name "COMBINING ENCLOSING CIRCLE BACKSLASH" (U+20E0) [2] and in emoji form it's the "NO ENTRY SIGN" (U+1F6AB) [3].
Those date from 1993 and 2010. There's also U+26D4 NO ENTRY (2009) and U+1F6C7 PROHIBITED SIGN (2014). I wonder what the history is that we ended up with four different code points for a similar concept.
I included just the ones that were the same visual representation, U+26D4 is a different symbol. The glyph for U+1F6C7 seems to be missing in most places, I can't find an authoritative representation of it to confirm it's the same visual symbol.
EDIT: Looks like U+1F6C7 "PROHIBITED SIGN" is indeed the same symbol, as seen in the Unicode 7.0 charts [0]
> I included just the ones that were the same visual representation, U+26D4 is a different symbol. The glyph for U+1F6C7 seems to be missing in most places, I can't find an authoritative representation of it to confirm it's the same visual symbol.
I don't think that there's any meaningful sense in which two Unicode code points may be said to have, or not have, the same symbol, except in reference to a particular font. The standard just indicates some possibilities:
> The shapes of the reference glyphs used in these code charts are not prescriptive. Considerable variation is to be expected in actual fonts. The particular fonts used in these charts were provided to the Unicode Consortium by a number of different font designers, who own the rights to the fonts.
To be fair, two of the four discussed here (U+1F6AB and U+1F6C7) are early "astral plane" emoji just after Unicode gave up on Han unification as a plan and moved back to trying to better round-trip existing encodings. The existence of emoji in general is partly Unicode's mea culpa for attempting Han unification in the first place.
(It was an odd sort of apology, given most of the damage was already done and the astral plane should have probably opened up a lot sooner in hindsight, but it was a fascinating sort of apology in the way that emoji has done more for Unicode acceptance testing in first-language-English developed applications than just about any other block in Unicode history.)
0: https://en.wikipedia.org/wiki/No_symbol
1: https://en.wikipedia.org/wiki/ISO_7010
2: https://www.compart.com/en/unicode/U+20E0
3: https://www.compart.com/en/unicode/U+1F6AB