Hack them back if you want! It won't hurt the plant and it will look a whole lot better if yours gets a second bunch of growth once the weather cools down.

Be careful that you don't step on the crown or pull out new growth when you do it though - there might be some shoots just below the surface waiting for those cooler temps.

It might not come back until next spring, but it's pretty common for it to have a second growth spurt.

BP

Yup, I hack mine back severly after they are finished flowering. They always come back with some nice foliage but not the same size. In the spring they will be back to make you smile once again.