On the pads, or just the disc? If it's just on the disc you should be able to clean it with a strong solvent like acetone, lacquer thinner, etc. I personally use denatured alcohol to clean my discs. Don't spray these solvents on! use a rag. Luckily, there are worse things you could have gotten on there.
If it's on the pads too... If it were me, I would try removing them and soaking them in DA to break down and lift out all of the WD-40. Then let them dry completely and try them out again under safe conditions. Othewise, just replace the pads.