The short answer is you can't. There's really nothing proprietary about what your doing. Short of your clients copying word for word documents, books, advice that you offer, there's nothing stopping them from taking what they learned from you and using that to setup their own business. Heck, depending upon the type of coaching your offering it could even be a scenario where you're teaching people how to get started in the same business as you and then getting upset when they actually follow through and do so.

I would suggest not worrying too much about your competition and focusing on the value that you add. There's only one you, only you have your knowledge and experiences. Use that and quit worrying about what others are doing.

Now if someone is copying materials you hand out that is something you could pursue, although you're likely not going to sue anyone for anything, it's more a hey quit doing that type of letter your attourney can draft for you.