Repeatedly tilling the soil is the solution, Alex. Sunlight kills them. So bringing them to the surface is the fix.
EDIT: If you were in a hurry or didn't wish to till over and over, digging the soil out and fanning it across a tarp for a day or two sounds like a fast fix.
I've also read in these forums about people that have moved a peony 2 or 3 times and can't get it to grow well. Maybe a water treatment is in order since typical methods are not working.
