Re: how to kill black locust stump?
What does having the stump next to the house have to do with not being able to dig it up? Sure, it will be more work, but you can still remove it or most of it without disturbing the soil under the foundation. Even if you do go under the foundation, undermining a 2 foot section isn't going to harm the integrity of the foundation or footing, just don't dig out any more than that and you'll be fine.
A stump grinder should be able to get most of the main stump and certainly anything that is out away from the house, which would save you the work and effort of digging at all. Once ground, remove the chips from the hole and dispose of them, back fill with native soil and you're good to go.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!