I hate them as they devastate my beetroot (I prefer to eat the leaves than the root). I don't think you can stop leaf miners with chemicals, as they live inside the leaf so are protected from anything you can throw at them. The best way I have found is to disrupt their life-cycle by rubbing out the eggs (typically on the back of the leaf) every few days. This keeps their numbers down but never seems to quite wipe them out, as I must miss a few.