You spray them with round up twice a week? Round up can take several days to work so how can they come back within hours of spraying them? Are you sure your using the right round up? 1 or 2 apps should rid of them as long as they are actively growing.

Not sure of where your located but if your in the area where cool season grass grows now is not a good time to seed. Wait until at least mid august.

Maybe I was being a little dramatic when saying comes back I a few hours, but I've sprayed at least 5 gallons of round up over the last few weeks trying to get them all killed. I'll spray and they will start to wilt and turn black and within a day the entire area is covered again.

Try a mix of triclopyr and glyphosate. I think RoundUp makes a brush and ivy killer that has those exact ingredients, check Home Depot.