Hire a tree stump grinder and grind the roots out, or hire a man with a grinder to come and do it for you if you can't dig them out. Tree surgeons usually have someone who will do this work.

If you do the work yourself, try digging a trench either side of the hedge, as deep as you can go comfortably, cutting through any roots as you go, then fill with water,let this soak away and repeat, the idea is to make the ground as soft as possible, then just dig and pull until they come out. it is hard work, this just makes it a bit easier, GOOD LUCK