The Mishna (Ma'aser Sheni 5:1) instructs us to mark vineyards as orla with burnt clay so others will avoid eating or benefitting from the grapes; Rabbi Shimon ben Gamliel limits this to shemita years. The Rambam explains that marking is necessary during shemita years since the fruit is ownerless, so anyone might rightfully partake of it. During other years, however, this would constitute stealing. If a thief does steal orla fruit, this is his problem (based on the halachic principle of hal'itehu larasha vayamot; lit. "feed the wicked man and he will die").
Nevertheless, it seems that one should pick the orla fruit from one's tree so that no one ends up using it accidentally (family, friends, neighbors, or other Jews). The fruit can be either buried, burned, or thrown away in the garbage.
From an agronomic perspective, it is recommended to clip the flowers during orla years to prevent fruit from developing. This avoids wasting the tree's energy and encourages a better harvest during permissible years. Of course, this is only if the tree is ours; we have no right to touch a neighbor's tree to "help him out" by throwing away all of his orla fruit!
