Be careful! The only documented chemical we are aware of that removes CYA causes more trouble than it relieves. I believe it's "Melamine".

You could use the "swamp" method to lower CYA. You let your FC go to 0, let algae grow and prosper for a couple of months, during which time it will consume CYA. Then you have to clean up the mess! (but the CYA will be lower)