You cannot prevent a former anyone except in many states of US via non-compete agreements. (not in california)

Non-soliciting agreement is a separate matter untested in court.

However, he was an employee at the time of soliciting, making it a very different matter. Directly prohibited by california work code.

so if someone leaves and starts their own company you can prevent them from actively recruiting former coworkers but you cant prevent them from hiring former coworkers who went to join them on their own without any active solicitation from the former employee, basically if the current employee contacts the former employee first then it is ok but contract forbidding the former employee from contacting current employees to recruit them is allowed. Also contract forbidding the active recruitment of current employees that the former employee never met are not enforceable so you cant forbid recruiting employees who joined after you left the company