Chemicals are not necessary if you just have a little diligence. New
sprouts just need to be removed after they push up a few feet, cut them
all off completely do this a few times and most trees will die. You can
also put down a piece of old carpet after cutting off the first load of
sprouts. Once the tree depletes is food reserve in the root it will
die.

Richard
http://www.arborsmith.com