I need to PERMANENTLY kill some vegetation. I know there's Roundup, but that seems to be "temporary". Is there something that can be applied to weeds/grass, so they DONT come back? I dont care if it ruins the soil, dont plan on EVER using this area for anything. Its rocked/matted underneath, but the stuff just grew up through the rocks It would be nice if its also "safe" seeins how I'm going to kill some weeds/grass here at work. Help!!!?????