How do you kill a tree?

You could cut a ring all the way around it through the bark. A chainsaw is easiest but a handsaw, hatchet or axe works too.

http://forestry.about.com/

You can chemically treat and kill trees

Go to Dr. Bonnie Appletons webpage and click on how to kill a tree -

http://www.treelink.org/docs/appleton_posters.php

Copper nails will do the trick also.