Olives are an important crop in southern European and Mediterranean countries such as Italy, but the European native species Olea cuspidata has escaped cultivation and become invasive in areas where the soil is rocky and summers are hot and dry. It's a problem plant in parts of Hawaii, New Zealand and Australia, where it overtakes environments where native plants struggle to survive. Humans, foxes and birds are responsible for spreading the seeds of this plant. One of the best control methods is not to add to the problem by purposely planting this tree.
Pull small seedlings by hand, wearing garden gloves to protect your hands. Then chop the seedlings into small pieces with your clippers to prevent them from re-rooting in compost piles or other disposal areas.
Spray the foliage of seedling trees that occur in a widespread area with the herbicide Garlon-4, diluted to a 5 percent solution. Repeat this spray after rain. Hawaii Ecosystems at Risk reports that foliar spraying has proved more effective in controlling this tree than manual methods such as pulling out seedling.
Cut larger trees to the ground with a chain or hand saw. Then paint the stump with an herbicide that contains glyphosate, the main ingredient in the product Roundup. Hawaii Ecosystems at Risk reports that a full strength spray of Garlon-4 and Tordon RTU has proven effective for treating cut stumps and is preferred over foliar spraying because the herbicide does not drift to non-target plants when used in this manner.
Collect all berries from cut trees instead of leaving them in the area where the tree grew because they will sprout and form more trees, subverting your control efforts. Place berries in a black plastic garbage bag and then dispose of them by burning.