Dry things out some if you can.  Go an extra day between watering.  Lots of air flow, the fly tape or yellow sticky traps will eliminate the adults. The larvae dry, dry, dry.  Get some diatomaceous earth and put it down to make a barrier like R's suggested with the sand, the diatomaceous earth does the same thing and kills any leftovers. Switch to coco coir and perlite. Go hydroponic, I did some years back and I have just had greaser buds and much faster growth, I understand the appeal of organic and if that's your jam, Rock it.