I guess it depends on where and how much is on the box. The shell of the box can probably be wiped off with bleach, I am not sure it could be machine washed.
If it is on the diffusion fabrics, and if they are our foldable boxes, I would just buy new diffusion panels. New panels are $7 to $17, dependign on size, and that is for both panels. You will need to call for these.