How big and how many? Surrounding conditions?

If you can cut them off straight Round Up on the stump imeadiatly will kill them.

Prodiamine (Active Ingredient) is a fantastic product. I have been using it for a long time. Always mix it with round up to kill anything you can see and the prodiamine does a great job of getting what you can't see. Depending on the spot you can get 2-3 years of control. There isn't anything that is environmentally safe that will do what you are asking for.

Not big and easily cut with a ripsaw. Thanks ! I can operate a Round Up !

Mohave is a wettable powdered that works great. As soon as it dries it bind to the soil and will not migrate like most other sterilant.

Cut them off and drill holes in the stump and fill the holes with borax.

Didn’t think about it but this is the best suggestion.