Unfortunately I have had the displeasure of doing this job, replace backyard with 100+ acres though. Main method was cut them off at around 200mm from ground level and then inject each sucker with a mix of kero and diesel, or just diesel. Under a shot per sucker does the trick. If not just paint them with diesel. Other method was not to cut them down but to give the ones you don't want to keep a small (1ml) dose of 24d (or 245t back in the day, mix them 50:50 for ultimate death). If you go this option you MUST keeps your hands clean and do not wipe them on any plant you want to live.