You have received a lot of replies. All of them most likely work. The roundup with the purple cap is very expensive. Some of the other products listed are probably cheaper and does the same thing.
A stump I want to kill I will cut the tree down in the spring or summer, and leave a 16 inch stump. Come fall (October)I will go out and cut the stump off and put a 50/50 mixture of the round up with the purple cap and diesel fuel. The park service uses antifreeze and roundup 50/50 in the fall and into winter on their buck thorn.
In the fall most ,if not all plants sent to the roots everything they suck in.