It is rare that for an employer to instruct its employees not to try to lure aware a competitor's customers. It is rarer still when an employer fires an employee for doing so. These may be rare events, but apparently (or at least allegedly) they have happened:
Amidst a corporate merger, a sales executive is told there are limitations on how he can compete for the merging partner’s clients. He loses sales commissions and is terminated for poor sales performance. Does he have standing to assert a cause of action under the Cartwright Act, California’s antitrust statute? (Bus. & Prof. Code, § 16700 et seq.) On the particular facts alleged in this case, the answer is clearly no.