I suppose that brushwood killer might be the answer, but you'd have to protect any other plants near them.

Use a serrated knife such as a steak knife and saw down into the ground in six inch blocks .After the removal of the first block which is tricky, try using a hand trowel,the rest should come up easily.this works for me .As i'm lucky enough to have access to molehills I use this for topping up.

Could you get hold of a matock, makes life alot easier, i use a matock if i have to lift pampas grass which can be very tough but always works well.