We use a product that comes as a concentrate. It is called Killzall, and works very well. It is much more cost effective than Round up.

Good luck! =)

I use white vinegar, straight up; cheap enough if you purchase at Sam's Club, Costco, BJ's...

Over time dust from the air and bits and pieces of organic matter will accumulate in those cracks and that is most often how "weeds" start growing. Spraying the "weeds" that are there now may well kill them but the dead material, if left in place, contributes to what future weeds can grow in.
The best way to eliminate weeds growing between bricks, paving blocks, etc. is to pull them up, clean out the accumulated soil, and then replace what was removed. That often gives you several years before you see "weed" growth again while spraying will be an annual project.

I made a mixture of vinegar, salt, and dish detergent, which works quite well, and quickly. I can't remember what proportions were, but put that in spray bottle with water, and kept it handy!

yeah, vinegar and/or boiling water. good for the environment, cheap and easy. Then scrape them out, I can't guarantee something won't grow in there later but that gives you a start.

PL