Ask Mr. Smarty Plants

As a last (and effective) resort, use a herbicide.  You must cut as many of the growing shoots as you can very close to the ground.  If there are lots of shoots, cut them in groups of six or eight.  Then, within five minutes at most, use a small brush to paint the cambium layer (or whole shoot if less than 0.5 inches in diameter) with CONCENTRATED Roundup herbicide (not the diluted form sold to spray on leafy vegetation).  The cambium layer is the ring of cells just beneath the bark.  It's functions include transporting nutrients made in the leaves by photosynthesis down into the stem and roots and transporting water from the roots up to the leaves. If the herbicide is not applied within a very few minutes after disrupting the cambium it will not be taken up effectively by the plant.  Then cut and treat another group of shoots and continue the routine until all shoots are treated.  If some of the saplings are as large as small trees you can simply girdle the trunk and apply herbicide in the cut opening.

Roundup concentrate can be obtained from some garden centers or hardware stores.  Roundup persists in nature for only a few days, but it is toxic to some animals while it lasts.  That is why you must carefully paint it on with a small brush rather than spraying over a larger area.  And do not use it within a few feet of a stream or pond because rain could carry it into the water, where it would kill aquatic organisms.  This is why we only recommend its usage in unusually stubborn cases.