Basically, we all try and provide the perfect enviroment for our beloved ganga plants and in doing so, provide a lovely environment for the mites as well. Do you grow anything else other than ganga in a perfectly tuned environment? I know I don't. But if you did, those plants would get mites too. I bet ya.
With regard to fighting them. I think I have a good trick for you to try. It worked for me and my mites have not come back this grow, yet (touch wood), for the first time in ages. I would normally be seeing mite damage by now.
This method will only work if you can have a days downtime in your room with no plants.........it may not work for your area, I don't know how your setup exactly......but here's what I did......
Remove plants
Clean room and equipment very well, don't bother with foggers or bombs. They are NASTY and don't kill mite eggs, as you have discovered.
Turn off all fans and ventilation
Turn on HPS lights
Spray 'Techno Spray' (an ethanol based eco household cleaner) all over your floor, walls, trays, anything in the room that isn't electrical....close up your room and leave it!
It will get VERY hot and VERY humid. My room was something like 60 deg C 80% RH. Mite eggs will POP! Check back an hour later, most of the spray will have evaporated and the RH will be going down. Spray again, all over. Shut room and leave it again. Check back an hour later. When the 2nd batch has evaporated, turn off your lights and turn on you ventialtion again to remove the heat. I think you will find your spider mite eggs will be dead now.
Continue with your Neem treatment going into your next grow for precatuionary measures. Until first few weeks flowering. That's where I am now, just stopped the Neem about a week ago. No mites or any other bugs/pests in my room.
This method was recommended to me by my local hyrdro shop. I went to buy a fogger and asked if they were good for mite........they said no! Don't buy it......do the above!
If you check out my journal, you will see me describe all this in detail with pics. And you will also see the state of my room pre cleansing!
Good luck though man!
oh...I also fitted a particle intake filter on my inline fan this grow too. I'm sure that is helping a lot as well!