Posted: 25/04/2012 at 09:56
What you're suggesting isn't practical - digging up an old tree risks killing it from root damage, and cutting off most of its roots is likely to finish it off. Further, trees respond to root damage by suckering, so wherever you damage the roots, you will get more suckers (if the tree survives).
The way to treat suckers is to brush back the soil to expose the sucker, then either cut it very close to the root or tear it off. It is a perennial problem with lilac, but controllable if you keep on top of it.