I cut the tops of two ornamental pear trees with my 18" chain saw; however the stumps are larger therfore, I want to destroy the stumps and the entire root system. These stumps are over six feet tall.

How in the world can stumps be over 6' tall?
Rent a bigger chain saw for half day rate and cut them to the ground.
There are many ways to deal with a short stump, but roots are another matter. Ask at any garden supply or hardware store. If you really have to get rid of the roots too, hire someone with a bobcat or backhoe to remove all of it.