I'm not sure they are spider mites. They are very small, the size of a grain of salt. Also, on the picture it looks more like white fly.
Either way, Boston Ferns present some challenges with insect control. The fronds are numerous and delicate. Normal solutions like soap and water or neem sprayed on the leaves won't give you the 100% coverage you need to kill the adults. In addition due to the delicate nature of the fronds you need to wash any control off a few minutes after applying.
Due to this I recommend the easiest method. All you need is a pair of scissors:
- cut all foliage off at the base of the plant at the soil level
- remove all dead material near the soil where pests can hide
- reduce water
- move to a high diffuse light area if not already located in one
- wait for new growth, it may take a few weeks
- watch for pest problems and control as necessary
- increase water with growth but do not keep soggy