Blocking out the sunlight for a few months geneally gets rid of weeds. For this i like to use a larger sheet of cardboard - a larger cardbos flattened out and weighed down so it does not blow a way in the wind, by a few large stones or bicks. Another good material to use is the black landscaping fabric. - again pinned down securely so it does not blow away.
You can actually start a raised bed on top of grass by using these methods. The cardboard will biodegrade into the soil and holes can be poked in the landscape fabric to allow roots to grow down through it. To make the bed, just pile clean bedding soil on top, and plant.