Keeping the weeds out of the sidewalk cracks is an ongoing project! To make it easier, try these tips:
Take a gallon of vinegar, spread it around, and in a couple of days, You will be able to remove all the dead stuff with a broom! It works great, the smell disappears shortly, and is cheaper and better for the environment than a lot of the "name brands" you can buy!
Rock salt sprinkled lightly over the driveway or sidewalk will kill any living plant. Be careful not to sprinkle it on the plants you wish to keep.