My trainers sign non-compete agreements for a limited radius, in effect for 1 year after they leave our company. It also includes a non-solicitation agreement of clients and members. I haven’t had to enforce it yet in my current situation so I can’t really say.
I will say that in the past, managing in other locations, it is obvious when a trainer is about to bolt and take their clients with them. Big tipoff is their business starts dropping off as they start pulling their clients into their “private practice” or moving them to another facility.
The clients sign contracts that stipulate they are hiring the company (not the trainer). If a trainer were to coerce a client into breeching the contract or interfering with it in any way that would be pretty serious in my book.
I also try to integrate multiple trainers with the clients when practical so they have a connection other than the one trainer. This also helps when trainers need time off or emergencies come up for the trainer. the client gets to keep moving forward!
The manager can also develop a relationship with the clients by stepping in and helping out when a trainer can’t fill all the roles (nutrition guidance, movement assessments, anything you are better at then the trainers who work for you are). These can be brief minutes or a supplemental (free) workout or a few words about their progress before or after a session.