Ailanthus. The so-called tree of heaven. You'll need to get it and its roots out.

Or can use herbicide (weed killer) with a cut-and-paint method which works best with 2 persons cooperating. One cuts of the weed (small tree), the other immediate plaints the cut surface with herbicide.

The active ingredient triclopyr is very effective for this sort of project. It' available in various products among them Brush-be-gone (sp?) and Crossbow.