Flags

Ive got 400x400 grey flags all round my house, they are just bed down on quarry dust and sand. So much grass coming up through them its starting to frustrate me, I've weed killed them and scraped out with stanley knife and hoovered but still comes back...

Ive been toying with getting joints cemented but I'm not even sure that can be done as they joints have a tapered finish and there is small knuckles on the sides of the flags so i think these are supposed to be fitted dry. Can anyone confirm this?

Secondly once i go through the process again is there anything i can apply weekly that will stop grass,weeds from growing through, i understand weedkiller relies on green to be present to work but is there anything i can just apply weekly to stop it ever from starting...