use some sort of roundup type spray on the blades of grass... its a systemic so it will go down into the roots and kill it. theres many many names on the market now. buy an extra strength one and be prepared to apply a second time etc. since youve already chopped up the roots by digging then you should just wait untill their new blades of grasse peek out of the ground. If you are worried about surrounding plants getting hit by spray drifting thru the air then hold a large piece of cardboard between the plants while spraying.

Sue
"The one thing all gardeners share in common is a belief in tommorrow"