How to kill it?
The first method I recommend is to dig it up. Get a shovel and dig down to six inches below the soil surface and pull the entire plant out. Don't break the plant into pieces. Do not put it in your compost pile. Wrap it and throw it in the trash or burn it. The second method would be to use one of these herbicides: 2-4-D (found in most herbicides used for lawns), dicamba, glyphosate (use only on the weed itself) or mecoprop, but I wouldn't recommend that if you're need waterways or if you're strongly concerned about the environment.