What do you mean by turning the heater off? Blower, heat regulator to cold or the A/C button?
Normally the radiator fans will only kick in if either the engine is running normally and needs to cool a little whether or not there is Air Conditioning or if there is it will run more often as the AC system needs cooler air to work more effectively whether it be on a high or a low cabin heat setting.
To be more sure of your problem you need to actually look at the AC pump when you think it is on and running continuously. The drive belt turns a pulley all the time the engine is running but if you look closely you will see there is also an inner plate on the face of the pulley which cuts in and out on a clutch. This can be difficult to spot so the best way to do it is to switch everything off and mark the face of the AC pump pulley with Tippex, switch the engine on, then the AC and have an assistant tell you whether it is moving as you switch the AC off and on again. Give it a minute or two to kick in as it doesn't come on immediately. As the engine is running the pump should kick on and off. Another thing to do when eveything is switched off is to feel the front of the pulley. The bit that spins and controls the AC will turn independently if you coax it round with a screwdriver.
Hope that answers your dilemma.
