Preventive treatment with a systemic chemical pesticide such as emamectin benzoate can help prevent a tree from succumbing to emerald ash borer if treatment is begun early enough—before a tree is infected and if the treatment is repeated every other year. But the cost of each treatment can be hundreds of dollars, and in an area where EAB has already gained a foothold, your tree may be living on borrowed time no matter what you do. That said, a healthy tree that is treated every other year by a professional can hold off EAB for a considerable length of time—enough time for a replacement tree of a different species to get a good start, for example. But be wary of any business that guarantees they can prevent EAB on any particular tree.