The aux fans are controlled by several switches and relays. Have you owned the car long? Can I assume that this problem just started, recently? If this has been going on since you purchased this car, then someone may have bypassed a switch to keep the fans on, probably to correct a overheating problem.
If this problem just started, then I would check the switches first then the relays second. Start by turning on the car, but don't start the engine. There is a switch in the top of the A/C drier, (drier is located behind driver's side headlamp assembly, black cylinder) it's red and has 2 wires made onto it. Unplug one of the wires and see if it stops. If it does then that switch is bad and you will have to go to the shop and have it replaced, due to the fact that the A/C system has to be drained. The second switch is located 2/3rds of the way to the back of the head on the driver's side, between the air filter housing and the valve cover. This switch is blue and has 2 pins on top for the wires to connect. With this switch, it is easier to replace the switch to test due to the way it functions.
Next is the relays, if they are stuck in the ON position. I don't have a wiring manual at home to tell you which relays control the fan, so I can not help you there until monday
The most common reason (I see) for your complaint is someone bypassing a switch to cover-up a overheating problem. Second most common would be the switch on the head, which when it goes bad causes the fan to run at high speed continuously and can cause the A/C compressor not to engage.
Let everyone know what you find
Hope this helps