Good Morning Jane, 

Thank you for contacting the U of M Extension Service. 

Please click on the following link from the U of M Extension Yard and Garden website: https://extension.umn.edu/weeds/creeping-bellflower

This information states:

• Small sections of creeping bellflower can be managed by removing all of the plant’s roots; repeated removal may be necessary.
• Broadleaf herbicides may be effective on large infestations, but creeping bellflower is resistant to many herbicides.

I am not familiar with the effectiveness of Preen. You would need to READ THE LABEL on the product. 

Good Luck!