That's your problem..... the fan should spin free just like a house fan in the "Off"Position.
Just hearing you say it pops the fuse confirms my opinion..... It's a lot cheaper to get a fan for it instead of a whole new furnace
..... take the fan out and try to work it free with WD-40 or get the model number and order a new fan.....
Just for a test when you get the fan out put a new fuse in and turn the furnace
on for five seconds and the fuse should not blow.
use caution doing that test because you don't want the furnace
(which takes about thirty seconds) without a fan in place..... just a quick test to show the fuse doesn't blow to confirm the problem