A few years ago I planted a sumac in the backyard for the fall color. It loved my garden soil & sent out runners in all directions. I removed the main plant after two years and had to continue to cut offf any new shoots from the runners for another year. Pesistence is the key. Keep an eye open for any shoots and cut them immediately. The runners are fairly shallow and easy to remove from a garden bed.

I had the same situation when we moved into our house a few years ago. It took me quite awhile to get all the roots dug up and gone. Luckily, (knock on wood!) it hasn't returned.