Round Up also produces a product designed for killing Poison Ivy and other more hearty weeds/plants.
I had an issue with a bunch of Poison Ivy growing in the woods behind my yard that regular Round Up wouldn't touch. I sprayed the Poison Ivy version out there and it killed it dead, pretty quickly.
Also, check at Home Depot, or whatever store you're using, and see if they have a product called
Image. It's rather expensive (@ $15.00/16 oz.) but this stuff is touted to be able to kill your heartier weeds, including nutsedge, and the like.
Randy