I think i'd dig round the root ball of the established plant and then sink stiff plastic strip [bought as lawn edging I think] to stop new shoots running. then carefully dig up the runners.
cant think what else you could do.
i am sure some one else may have other suggestions.

Oh lord, this is a real problem. Without digging up the lawn, there's not much you can do other than to poison any growth coming where you don't want it. Restraining the parent plant by inserting a rigid barrier 18 inches deep all round might help, but it's probably too late because it won't stop the current runners from developing and spreading further. I have the same problem under a paved area, shoots popping up in any cracks present 8 feet away...

Once you contain the mother plant, continual removal of new shoots where you don't want them will eventually kill out the escapees, but that may take anywhere from 3-10 years!