Spreading rumors and causing people to prejudge and hurting someones livelyhood goes against the NAP
Though a relatively common belief, this is not true.
A's concept of B belongs to A, not B and is formed solely by A as a result of sensory perception and reason. C's false negative statement, transmitted to A could only influence A's formation of the concept B, not actually form (and, thereby, enhance or harm) it.
C's statement, true or false, positive or negative, cannot be said to be aggression against the property of A (namely, the concept of B) any more than B's statements and actions (true or false, positive or negative).
Likewise, if A can choose to not patronize B's business, the possible transaction is not some property owned by B. So, even if C's false negative statement influenced A's concept of B to the extent that A chooses to forgo trading with B, it is again A who could only possibly be harmed (maybe A missed out on a good deal). But, while you're dealing in maybes, you also have to consider possible positive effects (maybe A found a better deal with someone other than B).