This is mostly a guess based on what I've encountered and judging by your hub's appearance of not being too abused....but I imagine it's locked up due to someone using heavy grease on the driver/clutch threads and letting it sit to dry up. First I'd recommend taking the clip ring off that holds the drive side dust cover on, and the dust cover will pull off. Lay it over drive side up and douse the internals with penetrating oil. Let it soak up whatever it will through the outer bearing race opening and in between the hub shell and sprocket. This will help loosen any dried up clay-like grease, at least enough to make disassembly easier. Give it some time to wick down into the hub and then try to rotate whichever side will free up first. When removing the drive side cone, you may have to rotate it counter clockwise a bit(to where it binds) then rotate the sprocket and hub clockwise while holding the brake arm and cone still to undo the binding induced in the clutch/driver....and repeat until the planetary gears are disengaged (approximately 1/2").
Hope this helps!