I dug up a tree a year ago and the roots seem to still be spreading under ground. Every once in a while I will see it sprout above ground. I keep pulling it out but it comes back. How do I kill the root forever without hurting the grass?
Just keep using your lawnmower. Set the mowing height a little lower for the problem area. If you told us what kind of tree we could provide a better answer.
Put epsom salt on the roots. Multiple applications. Another option depending on there size is poke a few drill holes in the roots and poor in some high nitrogen fertilizer. The decomposers in the soil feed of the nitrogen and process the wood much faster then usual.