What I have found with stubborn disc squeal is that the pads are usually irretrievably scorched. I always use the same brand pad on all my disc bikes (Delta Aztec sintered). As long as the pad never gets overheated, I can run them down to the point where the pad spring gets pulled into the rotor, and never get a squeal. But a scorched pad will never stop squealing.
Replace and upgrade your pad.
Also, double check the alignment of the calipers. If you're running Avid brakes, just run through the alignment process.