Let them die back, the leaves help feed the roots for next season.
If they have seed pods on then collect and sow the seeds fresh. they germinate quite easily and flower within 2 to 3 years.

I usually clip out the flowered stems once they're finished, then allow the foliage to die back naturally.

I have always cut down the flowering stems and left the leaves to die back naturally but this year I may leave the stems on the plant to take advantage of the frost we may get this winter. The stems with, seeds heads, should look great if we have a hoar frost.

Do you feed them at some point.

RHS advice for optimum flower display is to feed once or twice a week from when they start putting out leaves up until the flowers form, using a balanced liquid fertiliser. I just bung a bit of Miracle gro soluble mixed in water on mine when I remember...

Great, thanks Bamboo.