Dig, dig and more dig. Any roots need to be removed. If you're talking about couch grass, it spreads by underground roots which pop up where you don't want them - in the middle of a group of plants usually! The best way of dealing with that is to dig up the plants, clean the grass out of them, dig up every last piece of couch grass roots and then replant.

I agree with Spritzhenry - Its a horrible job and has to be done constantly but after some time the soil will improve and the grass not be such a nusance - I also put plastic ( old compost bags cut in strips) round my veg beds - It help keeping the grass from encroaching.

sadly digging is the best option. although you can use herbicide it takes more than one application.