Non Compete
Although a judge would have a hard time enforcing a non compete with an at-will employee who was let go, they will usually enforce it against someone who quit. But you have to limit the scope of the non compete. You can tell them they can't go work in the place across the street, but not someone in another town that is not competing with you.
Non competes are looked at for limitation in location and time. Just keep them as limited as possible and you will have some teeth, even if small, for your document.
That being said, the legal bill from enforcement would not usually be worth it for a $7-8 employee. Plus, as cafemakers said, they are usually most effective as a deterrent.
I've considered this myself for someone I may take the time and money to train in latte art. There is a justifiable reason to keep them from training with you, quitting and then working next door. Or the company next door doing it for that matter. No one in this area does latte art and it is something that would stand out.
That's my two cents