Hi horseball, and welcome.
I wouldn't worry too much about the black locust roots. Locust is known for being termite and rot resistant in damp ground.
If you can cut the stumps low enough, you can bush hog over them 3-4 times a year, they will finally die in a year or 2. If not, you can apply a very strong solution of 2,4-D to the fresh cut stumps, and they won't re-grow.
