Don't cut it down until all the flowers have finished. Campanula Persicifolia develops its new flowers on the same flowering stem, to promote this, deadhead regularly. Just tie it to a stake for this season.Cut the stem down to the base foliage (which will stay on the plant through the winter) Instead of staking next year try pinching out the new shoots when they are around 6 inches tall.This will provide a bushier plant that will withstand a downpour or two.
Are the stems damaged? If they are you will need to cut them down to the ground. If the are just looking a bit battered have you thought about providing some sort of support? I grow this and always find that I have to give it support once it flowers windy or not!