Cleaning Contaminated Disc Rotors
Isopropyl alcohol and/or brake cleaner from the car trade. Avoid washing up liquid in the cleaning water as it may leave a film of water dispersant. Don't forget the pads or replace the pads if they're too badly contaminated. Heating is a bad idea, it could damage the discs irreversibly.

To avoid contamination cover the rear disc when lubricating the chain and wipe off the excess of lubricant before the ride. Another source for contamination may be slightly leaking seals in the brake callipers.

But then concerning the lack of stopping power after new parts were fitted, did you break-in the pads and the rotors properly?