Dig the tree, prune the roots back.
Without digging the tree, use a saws all to cut them.
You really don't want big heavy roots. They'll always be big and heavy. You want fine roots coming off the trunk.
The root work should have been done before it was planted in the ground. Develop a good root system first. It's much harder to fix bad old roots than it is to proactively create a good root system when the tree is very young.
Most people new to bonsai work the branches first. Branches are the last thing that needs to be developed!
Roots first, then trunk, then branches. Take the time it takes to save time!