If they have finished flowering just go over them with the shears cutting off the flower stems but trying to avoid going too deep into the foliage. If there are still buds trim just above the buds - but this is fiddly so you could just wait until they are all finished. It really isn't such a precision art. They'll be fine.

Oh, I'm a fiddly Dianthus deadheader. :) I pick off the individual spent blooms and have continuous blooms right up until Fall. They really are a nice plant.