Legal Question in Employment Law in California

California has a strong policy against broad enforcement of non-compete contracts. Unless you signed a specific contract which obligated you not to accept employment with this specific category of client (i.e., three times removed), you are in a fairly strong position to object to and refuse the demand of your prior employer. Of course, there is no means by which you can simply make them shut up and go away. They have a right to make their demand, but now it is up to them to enforce it and that is likely to prove difficult. You could seek the limited service of an appropriate letter refusing their demand, and stating the legal reasons for such refusal, from a local labor or employee rights attorney. Or you can simply wait and see whether their demand ripens into a civil suit. In all events, you have the advantage of legal presumptions on your side of this dispute, so this is not something where you should be fearful or desperate to resolve.