Either dismantle the clutch or shock it free. There are various ways to try and free a seized clutch, but the one which I use involves getting the car onto a empty lot, and using a floor jack to raise the driven axle a couple of inches off the ground. Get in the car, get your assistant to check that the wheels are still off the ground, and start the engine. Let it warm up for a bit if it's a cold start. Then select top gear, raise rpm to about 2500, and on a signal get your assistant to suddenly release the jack lowering valve whilst you're pressing the clutch pedal all the way down. The sudden contact with the ground, plus the flywheel torque, is usually sufficient to break the corrosive adhesion. On a front wheel drive car, you must select reverse gear instead otherwise you'll run over your assistant!

There are other less violent and safer ways to try to free a seized clutch, and there's no point in me re-writing what others have posted online, so just search "freeing a seized clutch".

The adhesion is most likely from rust on the input shaft of the tranny and not so much at the flywheel. You need to carefully seperate the two without scratching up the flywheel surface. You really need a strong plastic (non-scratch) pry bar. First thing is some PB Blaster on the shaft splines.

You are either going to have to physically shock it loose or as said replace the clutch.

Doubt much else but replace the entire clutch....