The fungus needs wood/roots with intact bark to survive, so digging the roots out is about the best you can do I think. Pick all the mushooms as small as you can and seal them in a plastic bag so they don’t make spores. Getting a fungicide absorbed into those roots would be impossible I think.
The fungus can live a very long time on roots underground, a maple tree that was killed about 10 years ago sprouted honey mushrooms out of what was still left of the roots when they were cut to dig a hole to fix a water leak roughly 30 ft. from the stump.