I recently contaminated the disk on my downhill bike with wd40, a bad thing trust me when you don't realise you've done it. Don't ask me why but I didn't check before I went on a run and ended up hitting the ground hard when said brake failed to work. To remedy it I just cleaned the disk with white spirits and slapped in a new set of pads. A mate of mine had problems of getting his pads contaminated often, not sure why. He did some research and found that if you leave the contaminated pads on a cooker, it actually burns off the oil. Makes a lot of smoke though so you'd want an extraction fan. It worked for him. I accidently sprayed my disk again while trying to loosen the axle 3 days ago, couldn't be bothered to get out the spirits again so I just went onto the road and kept cycling fast then breaking, until the disk was really hot, then sprayed it with water, problem solved. Haven't had any problems with the disk on my trials bike yet.