Have a tree in my side yard that is repeatedly coming back, so I need to permanently remove it this time. What can I put on it that isnt toxic to the rest of the yard and its critters? It has 7 or 8 small trunks (finger diameter) and is Not a sucker of another tree. It may be Bois d'arc tree.

Take an axe or hatchet and chop the devil out of it. Not sideways but rather vertically, like you were trying to split each stem right down the middle. It should look like spaghetti when you are done. Cover this pasta with a sugar sauce of molasses, old jelly, granulated sugar or anything sweet you can find. Cover up with mulch and fagetaboutit.
Tony M