You could dig them up and divide them. Now is the time. The best divisions of large clumps of ornamental grasses are usually along the outer diameter of the crown, so you should take your divisions from there. If you want to keep the grasses where they are, dig out all except a smallish clump at one edge.

A better bet might be to replace them with something more size-appropriate for the space. There's a smaller cultivar of Pampas Grass called Cortaderia selloana 'Pumila.'

Does the pampas grass Pumila die back in the winter? I hear it is cold hardy in zone 6 and am anxious to try it.

Yes it does. It's supposed to be left standing, though, and cut back to a few inches in early spring.