Heat, lubricant and gentle persuasion
When the clutch release bearing fork is stuck the lever arm, use some heat and PB blaster, along with a gentle application of rubber mallot on the arm (using the case as a back-stop to the fork) to try to get the fork break loose.
Be careful though, as the little cast aluminum hoop at the bottom of the case that captures the end of the lever arm is fragile and prone to breaking if you are prying or smacking the lever.
-M