I'm not specifically familiar with your regulations, but there is one weed preventer that is considered "natural" that sometimes passes muster when others are banned. It's called corn gluten and there are several brands on the market in the U.S. and (at least formerly) in Canada. Check for that. It it's not allowed, you don't really have any options, other than keeping your grass as tall and thick and healthy as possible, which will reduce -- not eliminate -- the weeds. Then pull by hand as much as you can.