Good evening and thank you for entrusting me to answer your question. I will do everything I can to assist you.
Generally, the sort of agreement you have described would be unenforceable pursuant to Business and Professions Code section 16600, which states: "Except as provided in this chapter, every contract by which anyone is restrained from engaging in a lawful profession, trade, or business of any kind is to that extent void."
Quite simply, if the agreement prohibits an employee from "engaging" in that same industry following termintion of employment in any manner, it falls within this definition. Restrain on the ability to do business with former customers clearly falls within this definition, and numerous California cases have expressly held so. See here for a recent case in which the court invalidated an 18-month non-solicitation restriction on a former employee of a tax services company that prohibited the employee from providing services to any of the former employer's clients for a year: http://scholar.google.com/scholar_case?case=3664324957067638309&hl=en&as_sdt=2&as_vis=1&oi=scholarr
There are a few VERY narrow exceptions to the general rule that restraints on competition are unenforceable. The exceptions are a bit complex but generally involve instances where a business owner sells their entire business and agrees not to start up a new operation that directly competes. There is also a limited exception for "trade secrets."
However, a general prohibition on doing business with former clients would almost certainly be unenforceable. Accordingly, an employee under the circumstances you describe would not typically be violating an enforceable contract by dealing with former clients. Thus, such employee could freely solicit former customers without fear of a legal judgment being obtained against them, notwithstanding that an agremenet providing to the contrary was signed.
Please do not hesitate
to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes to you and thank you so much for coming to Just Answer.