Snipping is the way to go there. You can't use weed killer on/around something you're going to eat, and in any case it would also kill the chives.
If you're really persistent and snip off the grass plants as low down as you can every day or two, the chives will take over and the grass will die out.
If you're too impatient for the snipping method, the only other way to do the job would be to dig up the whole clump of chives, hose off the dirt, pick through to get all the grass plants out, roots and all, then re-plant the chives. It's easy to tell the chives from the grass roots because they have a sort of a little bulb - and of course they smell onion-y. You could divide the clump at the same time if you were so inclined.
