Is there a name for the common sign that signals a prohibited action;
i.e. a burning cigarette, with a red line drawn through it, that
indicates "NO SMOKING", or, on the Freeway, a depiction of a bicylist
with the red line which clearly says "NO BIKES". We see these signs
all over, in our offices, on roadways, in any country--one need not
speak the language to understand the message; the red line makes it
clear--DON'T DO IT!--But what do we call such signs?  Perhaps there is
no name given to them. Maybe, "Prohibitory Warning"

Hello, MileHiGuy!

I wish I could tell you that there is an interesting and entertaining
official name for these signs. (Negaslashes? Nuh-uh Notices? Nix Pix?
Diago-no-nos?) Unfortunately, the signs are codified by an agency
which, while it performs an admirable service in standardizing safety
symbols, is somewhat lacking in imaginative zeal.

The International Standards Organization (ISO) is responsible for
publishing the international standard ISO 3864 standards for safety
colours and safety signs. The red circle with a diagonal slash across
a black pictogram is officially called a "prohibition symbol," and the
sign which displays this symbol is called a "prohibition sign." Images
of this kind of sign, and other ISO-established signage, may be found
here:

Safety Label Solutions: ISO 3864 - Safety Colours and Safety Signs 
http://www.safetylabelsolutions.com/Standards/International/international.html

"Currently, the ISO 3864 safety colors and safety signs standard sets
forth a vocabulary of surround shape and color to indicate the type of
safety sign. In Figure 2, the solid blue circle with a white symbol is
a "mandatory action" safety sign, the red circle with a slash and
black graphic indicates a "prohibition" safety sign, and the yellow
triangle with a black band and black graphic defines a "warning"
safety sign. In this way, the combination of a symbol within one of
these distinctive colored shapes creates a safety sign in the
international vocabulary of signage."

Compliance Engineering: Safety Symbols
http://www.ce-mag.com/archive/02/03/peckham.html

"In short, ISO 3864 defines a vocabulary of shapes and colors to
communicate various types of safety signs. In this way, a person is
able to distinguish the type of safety sign from a distance. On closer
inspection, the sign's symbol communicates the specific safety
message. ISO standards are currently based on the philosophy that
through training and time, people will learn the meaning of symbols,
even if the meaning is not obvious at first glance."

Weinig USA: ISO 3864 - The Right Standard For International Labels
http://www.weinigusa.com/safety/pg11.htm

The documents which define this type of sign, and other safety signs,
are available from the ISO, and can be ordered here:

International Standards Organization: Safety Colours and Safety Signs
http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=9454

My Google search strategy:

"safety signs" + "red" + "circle" + "slash"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22safety+signs%22+red+circle+slash

"ISO" + "3864" + "signs"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=ISO+3864+signs

Thanks for asking a very interesting question that turns out to have a
rather dull answer! If anything above is unclear, or if any of the
links do not function, please ask for clarification.

Best regards,
pinkfreud