It’s important to know that disc brake pads are porous, so don’t use any kind of solvent on them or the dissolved contaminant will just soak permanently into the pad. A good scrub with water and a clean brush might help.
But if you’ve got surface oil contamination or glazed pads (from building up too much heat before the pads are bedded in), then you could try sandpapering or carefully grinding the surface. Bear in mind that even the natural oil from your fingers can contaminate a brake pad.
More extreme methods include fixing the pads in a vice and carefully heating them with a blowtorch to try to boil off the contaminants (don’t let the pad get to glowing point) or baking them in the oven.
At some point, you may need to just give up and replace your pads, especially if contaminants have soaked in.