My general approach would be….

Remove the spindle and strip of all removable parts.

Apply some heat, not too much just enough to get a bit of expansion of the spindle.

Apply heat FAST so that collet remains cooler than spindle. That drives the differential expansion and ‘cracks’ the bond that is retaining the collet.

Apply a bit of shock. Soft or copper mallet, through the drawbar.

With some care, that should shift it….

If not some heat and water quench cycles can help.

But again, not too hot, probably no more than a coupe of hundred degrees, 300 at most, if you see any hint of straw colour STOP… ! That’s coming up to 400 deg C. You are unlikely to get any distortion or issues, but no need to go that hot…

All the best.

As Jason says but don’t give up at one tap if it doesn’t release it. Try say 100 light taps which can often work better than one big one. Also think about what is reacting to the force of the tap. If you have a faceplate or chuck that you can screw on to support the spindle rather than letting the headstock and rear spindle retaining collar take the strain.