The worst ones are those that grow by way of underground runners, i.e., couch grass or quack grass. Sometimes, manually pulling it can make it worse. Obviously, you can't spray it with glyphosate (RoundUp) without killing the good plant.
Before we go any further, allow me to clear up some things about herbicides. The world is at war with Monsanto, the creators of RoundUp, and I completely understand why. Designing RoundUp ready crops and spewing glyphosate hither and yon over them is nuts. I would never use any herbicide around where food is planted. Nor would I apply it where there could be danger of runoff or drift. I also don't approve of Weed and Feed type lawn fertilizers, preferring to spot treat with 2,4-D. But glyphosate, or any herbicide, if used properly, can cut hours from your gardening chores.
Glyphosate is a non-selective herbicide, meaning it will kill just about anything it touches. I use it in the spring to edge around my flower beds (never food beds), and in my gravel driveway. I am careful to always apply it to the offending plant leaves with almost no contact with the ground.
So back to the grass growing in the flower beds. Only a non-selective herbicide will kill it. So, how do we get it on the leaves?
Enter the lowly Dollar Store kitchen tongs with some old kitchen sponge glued on the ends. I flatten the tips slightly so that the sponges completely meet when you squeeze the tongs together, and any excess can be squeezed off. I used Super glue because it holds up well when wet.
Grab a wide mouth pint canning jar and mix up a little glyphosate, and add some commercial surfactant to keep the solution from running. (Don't forget to suit up in latex gloves.) Take your newly acquired "Tongs of Death", dip it in the solution, squeeze together to remove excess, and grab the grass sliding it up the length of the blade. At this point, you may want to laugh hysterically, do a happy dance, and scream, "You're dead, dead, dead!"
Here's an area that I painstakingly hand pulled just weeks ago. Fat lot of good that did. This quack grass has completely invaded my ground cover.
I took these pictures about an hour after applying the Tongs of Death. Notice the blades are already tipping over. The shine you see is from the surfactant. Notice also, that little dead patch of ground cover. That's what happens when you try to spray herbicide on the grass with a hand held squirt bottle. Not my best idea.