If it's to kill everything (i.e. patio stones) and there's no grass or vegetation nearby, then Round up is a good bet. The "Good" Round up (much more effective version) is kept locked up and you have to ask for it. If your stones are located near grass, I'm stumped. All of the good stuff is banned in Ontario.
Herbicidal vinegar works almost as quickly and effectively as round up. It is sold in most garden centers. Landscape suppliers may sell it to you in concentrated form which is very effective.
Propane torch.
There are even special extension attachments at garden stores so you can do it standing up.
This is how real men handle the problem...propane torch and a beer.
Environmentally safe also.
I have been using a propane torch for years. If it is small weeds between the cement paging block, an ordinary welding torch is all you need. Bigger weed need to be chopped off before you torch the ends. It works really well just after it rains. the weeds soak up water and the torch boils the water in the stems to steam them to death.