I have read that vinegar will work. I am planning to try it myself soon.
What kills weeds permanently?
Bring a solution of about 1 cup salt in 2 cups water to a boil. Pour directly on theweeds to kill them. Another equally effective method of how to kill weeds is to spread salt directly onto the weeds or unwanted grass that come up between patio bricks or blocks.
I use a spray weed killer with a hose attachment like this one... https://www.homedepot.com/p/Ortho-Weed-B-Gon-Max-Plus-1-33-Gal-Ready-to-Use-Crabgrass-Control-044701005/206803434?cm_mmc=Shopping%7CG%7CVF%7CD28O%7C28-1_CHEMICALS%7CScotts%7CLIA%7COrtho_Fire_Ants%7c71700000048556277%7c58700004828075514%7c92700043974336854&gclid=EAIaIQobChMIodCr8uSt4gIV1lqGCh0EawxREAYYASABEgLWQ_D_BwE&gclsrc=aw.ds
Water well and cover with black plastic. Let sun bake it for several weeks. Not only will it kill weeds, it will sterilize soil. I bought some irises which had a fungus and this was the only way to stop the spread. Don't use vinegar or salt because it will stay in the soil. You will end up with acidic soil from the vinegar and dead soil in which nothing will grow with salt. I even bypass the water softener when I water the garden. It makes a tremendous difference!!!
Try multiple layers of plain cardboard - like the shipping box. Weight it down.