HELP! Kill tree roots - leg size and partially above ground all over ....

We have a 30 year old Cottonwood that saw it's best limited life years ago. We let it be out of loyalty for the many years of nice shade. The roots of this tree are now to the point that we cannot mow the yard without great difficulty. We've cut out high points of roots for years, but they are way ahead of what we can do without destroying the entire rear yard. Our yard is so bumpy, the children have fallen over the roots while playing. All block walls have been repaired again and again due to damage by these roots. We give, we will remove the tree this fall.

We think these roots will have a life of their own for a long time. Does anyone know of a product that might kill some of the larger underground legs by drilling holes and filling with something? I don't want to destroy 30 years of other tree and shrub growth in the process.