You cannot enforce any fanciful methods of stopping a former employee. Why? Because one they have left your employment and therefore your unique operating methods, they cease to be a part of that enterprise.
So, just because your man was tought the trade under your instruction, unless he literally shows EVERYTHING that YOUR company does, he is operating as a free agent, and therefore is untouchable in law.
So, if you want to proctect your existing customer base, you must look closely at how you operate; for instance, how you speak to your customers, your appearance, the condition of your vehicles, and most importantly, the actual job that you are doing for that customer! I have seen businesses that have had huge sums of cash injected into them, with liveried staff, smart premises, etc but let down buy the actual job itself. that, my friend, is where you need to concentrate, not on what a former staff member does or does not do. Get YOUR business right, and you have nothing to worry about.