Nicotine, like pyrethrins, will only stun insects unless the strength and application are exactly correct (pyrethrins actually require a synergist to kill). I would go the ladybug/pirate bug route if possible, especially now that you apparently have knocked the populations down. A cheap fix are colored sticky cards, which we use in the greenhouses routinely for fungus gnats, whitefly and thrips. They don't do squat with spider mites, though. 2-spotted mites are almost impossible to eradicate once the population gets to a certain point. Keeping them from exploding might be all you can do at this point.