The reality is that spidermites are everywhere. You could never completely rid your grow room of spidermites and it only takes one pregnant female to start an infestation.
They key is in not giving them conditions in which they can thrive. Spidermites like high temperatures (above 25 c), they also like low humidity. So keep the temps low and the humidity high and you've greatly reduced your chances of getting an infestation.
If you can't do that, or the grow room is full of eggs from previous infestations, then I recommend a product called
Spidermite Control. It's not toxic to humans and degrades within a couple of weeks.
If you have an ozone generator, you can use that to good effect by flooding the grow area with toxic levels of 03, between grows.