How can you inhibit a dogs sense of smell?  Are there any plants,
herbs, spices, extracts, anything that will automatically inhibit a
dog's sense of smell?  I want something that will work instantly and
be effetive.

A little dab of Vicks Vapo-Rub or Mentholatum under the nostrils is a
good smell-inhibitor. People who must work around decomposing bodies
sometimes use this trick. Should work for dogs, too, although I would
expect that the dog would be somewhat annoyed by it.

Powdered dogweed works pretty well, but reeks to humans also. A couple
questions to clarify yours. Are you looking for something that
actually reduces the dog's ability to smell, or overrides it with a
different smell, as is the case with dogweed? Also, are you looking
for something to give the dog (i.e. to be ingested) or something to
put near what you don't want smelt?

British intelligence uses a mixture of equal parts caproic, I-valeric
acids, and castor oil. The CIA also has a simular mixture which uses
acids of lower volatility that is composed of phenylacetic and/or
n-caprylic. Hope this helps!

A 2 or 4% zinc sulphate heptahydrate solution has been used to
temporarily disrupt olfaction in animals for biological research.
Recovery time varies with different animals but is usually longer than
24 hours.