I have that pest Adenophora (looks a lot like campanula, but i guess it's a bit different).
I had a 2 x 2 m area with lots of plants. All I did was spend a lot of time, digging the soil with a spade (the one designed for going deep in the soil, pointed, not used for removing soil) and in each"slice of dirt" I carefully searched for all tiny roots and so on.
So far, a few months later, I haven't seen new ones. I am thinking they are not all gone, but I am sure this method works - with a lot of patience, and for small areas.
