It's a Buddleia, probably 'Honeycomb.' I'm not sure what shape you want, but if it's a nice, tidy, round, tightly-trimmed one, this is not the plant for you.

As flowers fade, you can cut it back, but this is essentially deadheading and it will stimulate more rapid growth.

In my area, butterfly bushes are commonly cut back to about two feet tall each year in early spring. You can trim back long branches throughout the summer if you like.

Severely. You cannot hurt this plant.