how do I get rid of this invasive weed??
Invasive it is! The individual stems are all attached to an underground rhizome. Pull the plant and the rhizome lives on. There will also be seed in the soil from past years. Remove the top growth and sun hitting the soil will germinate these seeds. Double whammy. RoundUp will kill it. If you don't want to go that route, remove as much as you can then cover the ground with layers of cardboard. Denying sunlight to new sprouts causes death of the sprout. Expect bellflower to reach past the cardboard and come up. You will have to vigilantly pull these - at least weekly. Eventually you will exhaust the rhizome. To make it look tidier, apply mulch over the cardboard. Since seeds live several years in the soil, removing the cardboard will cause some of these to germinate. It is a several year project to rid your garden of creeping bellflower. good luck