I'm assuming you mean controlling them now that they are already in there. Try making sure there are leaves in your culture for your isopods to eat. Don't feed anything for a week or so. You isopods will be fine as they will eat the leaves. Your mites will start to get hungry. Now pick a nice flat leaf, moisten it, and put a large, healthy pinch of yeast on it. Wait 12 to 24 hours and check it. I have done this to find hundreds of mites clustered all over the feeding leaf. Do this several nights in a row to remove the vast majority of the mites.
This will allow you to continue running this culture and feeding out of it until your new, clean culture is producing well.