After looking over some labels, it looks like the GRASS KILLER post emerge might be Okay to use over bluestem and forbes. I'm not 100% sure on this so you may want to treat a small area to confirm before doing anything extensive. But there are no warnings on the label and from the documentation I've found, it appears that any invasive grassy weed would succumb to the Grass Killer and most prairie grass is strong enough to handle this approach. But as I see it, treating before the grassy weeds arrives would be smart and some pre emerge would be ideal for this need.

The one I would recommend would be SURFLAN. It's active on most anything trying to grow from seed and 2-3 treatments a year should do it. I've got labels posted on these here:

http://www.bugspray.net/labels/poast_grass_killer_label.pdf

http://www.bugspray.net/labels/surflan_label.pdf

The products can be seen here:

http://www.bugspray.com/catalog/products/page2004.html

http://www.bugspray.com/catalog/products/page2005.html

Hope this helps.