Before you do Gryme's check, try this troubleshooting first:
(with the ignition on)
*turn the vent direction to anything other than defrost,
*press the recirc button to ON,
*turn the fan knob to OFF.
With those settings engaged, no air should be coming through the vents and the A/C should be OFF.
If your vents are set to defrost or defrost/bi-lev, the A/C will be ON regardless of the A/C button being pressed, as that is the factory default setting.