Lilacs are beautiful, fragrant bushes that are often planted as ornamentals. But when it comes time to get rid of a sick, declining or unwanted lilac bush, their less attractive qualities come to the forefront. Cutting a lilac bush is easy. Its roots, however, are much tougher to kill. To stop the lilac from resprouting, you'll have to deliver a dose of herbicide directly to the roots.
Saw the lilac bush back to within 6 inches of the ground. If you have already cut your lilac back, make another pruning cut at least an inch below the first.
Dip a paintbrush in a bottle of broad spectrum herbicide.
Paint the herbicide onto the cuts you made in the lilac's stump. Apply the herbicide immediately after you make the cut. The quicker the application the more effective it will be. If you wait longer than 30 seconds, cut the top inch off of the stump and paint immediately.
Monitor the lilac's growth. If you see any sprouts growing on the cut stump, cut the top inch off of the stump and paint again following the instructions in step three.