There are a few items on the market that will control/kill spider mites...neem oil and Bang! are just a few. Sunlight Supply also has a new product out (I just added it to my store). Here is a description from the supplier:
"SucraShield sugar-based Insecticide/Miticide/Ovicide provides fast and sure biochemical control on a wide range of soft-bodied problem pests in the greenhouse and nursery as well as fruits, nuts, mushrooms and vegetables. Based on natural sugar esters, its unique mode of action creates holes in the target pest, causing rapid death from dessication and suffocation. Not for sale in PR. EPA Reg. No. 70950-2-84710. "
It's new to me but I thought I'd mention it.
You might here some good and bad about neem oil. For example, it kills the mites but not the eggs. On the instruction bottle it will tell you to reapply every 2-4 weeks.
Coincidentally, I had a mite problem myself this last summer...September, to be exact. Ughhhh....what a hassle, right?
In any event, I completely drenched the plants (sorry, not marijuana plants) with a pressure sprayer I took from my shelf and then bleached the heck out of the room. Well, for whatever reason, two months later, I have no mites. I wasn't really expecting to eradicate the mites, just control them. It was definitely a bonus.
If you do decide to buy neem oil, don't buy the big container even though it's cheaper per ounce. A small bottle will last you for several months.
As for Bang!...I have used it in the past for customers but I don't think it's as good as neem oil.
Overall, don't sweat the mites. They are a problem but 100% completely controllable and you can still have the best crop ever!
Hope this helps.
(if you're using neem oil, the word on the street is it will produce less marijuana if used in the flowering cycle. I have no direct experience with marijuana so cannot validate this)