hit the side of the clutch while the car is on. If the compressor is deep in the engine, then you'll need something long like a piece of rebar of a long torque wrench to hit the side. You'll have to be extra careful not to get whatever it is you're using caught in the serpentine belt while it's moving. Just be super careful. It doesn't take much to tap the clutch in.

If the ac turns on then you need to shim the clutch. Sometimes the clutch shimming isn't too bad and the ac will work for a month before you have to hit the clutch again. I did that on my college car for about 3 years before it gave up the ghost.