Cut them down prior to them going to seed and then let them lie there as mulch/ground cover. A hand-held sickle can be very useful for that....
At least that's what I did last year as it became impossible to keep the grass (and some weeds) out of the garden by hand.
Just to qualify that statement, during peak growing season my yard can requiring mowing multiple times a week just to keep it from growing into a "forest" as the grass will grow more than an inch/week in good conditions. ...and that may be a conservative estimate.