Increase of humidity for a few days generally worked for me, 70-85% should do it, or buy some predators that feed solely on spider mites?...once grow has finished treat the whole growroom before replanting....good luck.
first, after all ur grow is done, dont start a new one. then clean clean clean. no organic material at all in the room. then, one by one, introduce sterile grow material. clean pots, clean soil, ... they dont have a long life so u are regenerating them continuously, so chem isnt gonna necessarily work. maybe they are even from another part of the house or if ur in the right part of the world, outside.