A fence surrounding the garden will discourage unwanted pets, dodgeballs, bicycles, and the occasional mischief-maker. A chain link or wire fence provides security without blocking light or air circulation.
A living fence can be created by planting vines or hedges appropriate for your climate zone. If you are starting from scratch, however, it will take a long time for this to be effective. Keep in mind this type of fence may also create unwelcome shade on the garden and roots may compete for water and nutrients.