You can try having employees sign a non-competition agreement wherein they agree to not open a similar business within one hundred miles of yours for a period of up to five years after they leave your employ. Keep in mind that legal challenges to these agreements have been made and the complaining parties have prevailed, at times.
Other than the above, a good sixth sense about people you hire comes in very handy. Go-getters who want to know everything during their first month with you can be a warning sign that you should watch how open you are with them. In my former profession as a tattooist, I saw shop after shop hire apprentices who, after their first six months on the job, left their employer and proceeded to open their own shop, sometimes literally across the street. The skin biz went from one where you could make $50G's+/yr. working a very easy work week (mine was a grueling eighteen hours) to one where tons of kids, 21-25, were working six days a week and pulling in less than $10G's/yr. because of the glut of shops in town. While the tree biz seems to have room for just about everyone these days, I'm sure that if more and more folks get into it, eventually the pie will be sliced too thin for anyone to make a good living, as happened in the tattoo business.
Ultimately it all comes down to there being too many darn people in the world today. Sooner or later, all businesses/professions get done to death unless they can figure out completely unique ways of doing things and then get legal protection to keep others from doing the same. As I posted in here a while back, all of us in the tree business today should push for ultra-strict rules and regulations to keep hacks out. The legal and medical professions are great at doing that, and that is why they can stick it to the teeming masses, daily, and live in their comfortable mansions.