I believe you can remove the stains with dry cleaning. I have done quite a bit of work with extremely finely powdered laboratory grade activated carbon (the activation process is cooking it gently with sulphuric acid, btw, though in fact it is quite activated just from low-oxygen roasting it over fire), it is extremely hydrophobic and sticks to anything that is nonpolar (plastics, oils), so nonpolar solvents remove it. thus, dry cleaning. Dry cleaning solvent is a medium chain hydrocarbon around the same length as kerosene, except with the aromatic benzene containing fractions fully removed. You could probably use odourless kerosene or zippo fluid to get it to come off, and obviously, a soak in a fairly strong detergent solution overnight will get a lot out as well.