AC won't shut off (w/car off)

[TillmanRoss]

My 2000 Grand Prix has a problem with the AC. Last weekend the fan went from where I had it set at level 2 to blowing at the highest level. It didn't matter if I changed the control dial to 1 or 5 it was still a constant high output. Changing from heat to cold works, but does not effect the problem. Also going from the front vent to the defrost to the floorboard vents had no effect. The strange thing is it continued to run when I shut off the AC and fan. Even stranger, it continued after I turned off the engine. Any advice is appreciated.