Hi Jo Before you go to the trouble of removing the roots, consider this. Now that you have cut the shrubs down, if you let them grow back and prune them annually, you may end up with a couple of very nice shrubs.
If you still plan to remove the roots, you may be able to wrap a chain around the stumps and pull them out with a truck. There are also products sold as "stump killers". These products are usually applied to freshly cut stumps and are absorbed and moved downward into the root system to kill it. These are strong products, so read the label in the store before you buy one, to see if you are comfortable using it.