I'm not familiar with the fan design, but if possible, set it up so the threads stay immersed in PB blaster for a couple days. I would assume you could point the belt shaft straight down, perhaps situate it in a bottle, and spray between the motor and the blade so a puddle stays there for a couple days. Even then, if it's really stuck, it will take some force.
Not sure of the best way to grip the fan.