Which switch off?
The A/C will run when the defroster is on without the A/C switch being on. It should not run even with the defroster on or the A/C switch on with the fan switch set to off. Yours runs ALL the time?
Maybe the AC clutch is frozen? If it’s frozen up, that would make the AC always be “on”.
With the truck off (obviously) try to rotate the AC compressor by hand….the center portion of the pulley is the compressor, the outside being the clutched part. It should rotate with a little bit of force. If the pulley tries to turn (with the belt) it’s locked up.
To see if it’s an electrical issue, start by unplugging the AC clutch.
Also, there is a way to have the AC only work in AC mode….not Defrost. (That is if it's similar to my '04) Pull the bezel off from around the heat/ AC/ radio controls and you'll find a simple switch on the AC/ defroster mode dial....just unplug it and the AC will only come on in AC mode.