I've cleaned pads that were generally new and that I suspected were contaminated (made a hell of a racket at low speed). Spray brake cleaner on them, then burn it off (using a long stem grill starter to light it). Obviously: do it outside, and use a pliers to hold the pad, because it will get hot. This has only worked if the contamination is light, when I broke a piston and saturated the pads, there was no hope. But, contaminants from the road or fingers seems to respond well to this. If you choose to do this, make sure you test the brakes before you set off to bomb down your local canyon descent.
I am comfortable doing this, and have had good results.