"It is important to note that just about all of these kinds of policies violate the Right to Work laws in California. Specifically, if you block a hire based on this kind of policy and the employee loses their job and cannot find work, your company is liable for his wages. As a result, the business relationship with the other company must be extremely important for you to employ any kind of “hands off” policy."
https://a16z.com/2011/02/23/is-it-ok-to-hire-people-from-you...