Cedar bush roots run deep. In fact, cedar bush roots often have to be killed because they run into septic systems or other underground pipelines. Root killer can be used to kill entire bushes. However, you don't have to kill your cedar bush to get some of its roots out of the way. Some root killers can be fed into a septic tank to kill cedar bush roots from the inside. Others can be inserted into the ground as a barrier to troublesome root growth.
Cut the cedar bush's foliage back as close to the ground as possible with an axe or saw. Coat the stump with root killer. Different brands of root killer come in different forms; spray, paint or cover the stump with granules according to the manufacturer's instructions. Apply the root killer again if you notice any re-growth. This method will kill all of the roots so you can remove the bush entirely.
Pour the manufacturer recommended amount of foaming root killer designed for septic tanks into the toilet or drain to kill only the cedar roots that are interfering with your tank and its pipes. One application should be enough, but follow the manufacturer's instructions for application and frequency rates. Once your septic tank is clear, re-treat the tank once annually to keep the roots from growing back.
Drill a series of holes into and around roots that threaten to grow into sewer lines, sidewalks or driveways. Fill the holes with root killer to kill the root and prevent it from growing in that direction. Or, dig a narrow trench between the tree root and the sidewalk. Fill it with root killer and top it with soil. Repeat both processes annually to stop growth permanently.