Ok . . . . so when you say "not turning", you mean you grab the clutch with your hand, and are unable to twist it? Does it move at all, or does it just feel solid? If you stick a bar of some sort, like a long screwdriver, through the bolts on the clutch, and try to turn it that way, what happens?
On a brush hog, like a 106, what happens when you grab the (blunt side of the) blade and try turning that?
In my experience it's pretty unusual for attachments like that to just wedge up tight. The gearboxes are pretty well sealed. If you've got that many of them, it seems hard to believe that they are all that wedged.
Perhaps the whole works got left out in the weather for years? Post some pix?