I just had to deal with these a$$holes on a friends grow, pyrethin based products NUKE them in veg, but in flower you're probably not gonna get rid of them, just keep them knocked back. I used Lost Coat Plant Therapy, it's a store bought bottle of citric acid, alcohol, soy oil and a few other things.
The absolute best thing I bought to get rid of them? An electric battery powered sprayer. I was not doing NEARLY a good enough job with a hand pumped sprayer. Mine was about $80 with a battery and charger at harbor freight, I have seen other ones for $40 at Menards/Walmart type stores.
I also bought one with a metal wand so I could put more of a bend at the end of the wand, hold it low and spray upwards since the spider mites only seem to hang out on the bottom of the leaves.