Mature olives and other trees usually flower and produce fruit every year. If this characteristic is undesirable, you should consider removing the tree and replacing it with another species that does not have this characteristic.
Olives and some ornamental trees can be sprayed each year to reduce fruit production. Florel® brand Growth Regulator is registered to reduce or eliminate undesirable fruit development on many ornamental trees and shrubs. A list of specific plants which can be treated and more information about the product can be found online at Monterey Lawn and Garden Products Company. The product is sold at some retail garden centers. You may be able to treat a small tree yourself. Special equipment is required to treat large trees and the services of a commercial pesticide applicator is usually required.
Florel® brand Growth Regulator must be applied at the mid to full bloom stage prior to fruit set. When applied to plants, it enters the plants and breaks down to ethylene, a naturally occurring plant hormone. A sufficient amount of ethylene will cause developing flowers to drop. The diluted product does not harm bees. The concentrate is very acidic (pH 2.5) and protective clothing including waterproof gloves and goggles should be worn to avoid contact with skin and eyes. Avoid spraying on shiny painted surfaces such as cars and trucks as this can damage paint.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012