Try full strength 6% heinz vinegar sold at walmart.................if that does not work, they have one chemical that kills roots.........I don't like to use that because of pets & children............we took a popular down 2 years ago............we keep cutting the shoots off...........drill some hole in the tree base............if you have a flat rock big enough to cover it to not let it get air & water or have someone come & cut it out with that special unit that looks like a pie cutter. That will kill the roots.