To answer your original question--a good way to kill any tree is to place an impervious barrier (plastic, concrete, etc.) under the drip line area. Doing so deprives the roots of oxygen that they need. With trees as large as yours, stay at least 5 or 6 feet away. Of course, a wooden deck is a different matter, and can be built within a few inches of the trees' trunks. Expect to encounter serious roots when excavating for the deck's footings, though.