Although I am a newcommer germinating for my 1st grow and have never tried this on MJ, I have used this method for years on all my garden vegetables with good results.
By some pipe or rolling tobacco (NOT ready made cigarettes) and soak it in water overnight. I use about half an ounce (12g) in half pint. Bring it to the boil, strain and cool.
You will end up with an unpleasant brown liquid that is lethal and organic. It's a strong poison so make sure nobody drinks it!
Mist it onto your plants and watch everything with legs die.