Air Con not turning off

[tsopanos]

Hi,
I have an E36 that decided that the Aircon was never to turn off again!
I use to hear the engine change idle speed when I pushed the Aircon buttton, but in the last month I've noticed that even if the Aircon is off and I've got it on circulate, (or not) the air coming out of the vents is freezing. I then hit the Aircon button and there is still no change....it's still freezing.
Another thing that makes me think there's no change is the fuel consumption.
What could have caused this??
Greg