Use a shovel to dig a trench around the large plant or tree -- typically about 1 foot away from the stem or trunk and about 1 foot deep. Dig deep enough until the roots are exposed. Then keep digging outward, to make sure the trench is wide enough around the exposed roots so that you can safely operate your electric saw to cut through them.
Cut all the exposed roots with your reciprocating saw. Reciprocating saws use hardened blades which are able to cut through thick roots. Other electrical saws will also work on certain types of roots.
Remove the roots with a shovel or your hands. It is important that you make sure you get rid of all cut roots so that they do not begin to grow another tree or plant.