There are many types of campanulas or Star of Bethlehem, including some that can become invasive and root via the stems and also drop seeds. If you want to control it, keep pulling it up and cutting it back. Then mulch with wood chips on top of the soil to smother any seeds. Hand pulling is best around other plants, and this is easiest after a rain the soil is wet. In an open spot you may be able to use Round Up in late spring when the plants are actively growing to kill the tops as well as the roots. Good luck, Marianne Binetti