Poor soil quality is the source of the problem. However, replacing the soil is a drastic solution that will probably kill the plant as well as the critters. If you repotted your Crassula, then it is probably in a pot that is too large, in which case you can consider removing excess soil from around the outside of the original rootball, but leave the original rootball intact.
In any case, remove any soil from the surface that is not in immediate contact with the roots. The top layer of soil is where many of the larvae live. The excess top soil also keeps the soil in the root zone from drying out properly. Starving the larvae of moisture is the key to control. For that reason, I suggest you stop spraying and adding any liquid substances to the soil. Crassulas don't tolerate constantly moist soil, so it will benefit from your allowing it to dry out.
