No matter what restrictions and rules you have in the contract you can never trust someone 100%. Vetting them from the beginning and making sure these are people you can trust and work with is a good way to start from. You can do this by Doing a background check and calling around their references. Even if you offer them larger % of revenue you can never be 100% sure they will not steal business from you.
Having a non compete clause in the contract helps, but it might not hold up in a court of law. It will have to be a non solicitation clause (meaning they can’t solicit any of your clients) once they don’t work for you. Of course the longer they train these clients the stronger their bond with them will be. And even if they don’t solicit your clients, nothing stops these clients to seek them out after they are gone.
The best way to protect yourself and your clients from something like this happening is to keep contact with them at all times. The best way to do this is by training them at least once every few sessions to see if they have any issues or concerns. I know many business owners who did not keep in touch with their clients and they lost that connection.