Given you read the instructions, and Jonny's post, you know to turn the bolt clockwise to remove. Even armed with that info the bolt can be hard to remove so don't be afraid to really give it some torque.
First time out I had to use a cheater pipe on the end of the hex wrench. No problems since then as I make sure not to overtighten.
Tom is right to suggest an impact gun. Sometimes the hammering motion will break a bolt free that resists the straight torsion exerted by a wrench. Just be sure to have it set to turn the correct direction. Righty Loosey in this case!
Just be careful that you don't under-tighten it either. It is not a self-tightening bolt if the blade slips. The flanges are keyed to the shaft to allow the friction pads to properly grab the blade. The spring washer on the outboard flange provides the proper compression on the blade. So you just need to make sure the bolt is tight enough to not self-loosen with usage.