Posted: 02/03/2016 at 07:33
this is a naturally suckering tree anyway but cutting it back simply makes it go haywire as you have now discovered.
every shoot you see needs to be sprayed with strong stength glyphosate but not in the lawn. In the lawn a combination of lawn weedkiller and mowing....I would use secateurs or scissors to cut every shoot seen to ground level and do this as often as you can.
this is definitely not a plant for the garden. Never be tempted even by its brilliant red/yellow/ orange leaves in autumn