Q.Purple bellflower
How to prune? Do I just remove the spent purple flower or cut off the hard calyx?

When you deadhead, you pinch the flower on the stem beneath the calyx, removing the entire flower head. For more information on deadheading, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/fgen/deadheading-flowers.htm
For more information on the care of bellflowers, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/campanula/growing-campanula-bellflowers.htm
Happy gardening!