Sometimes as bg said, an impact is the only way to go.
I use an electric impact gun when I'm away from my shop.
You can pick them up pretty cheap on eBay or at Harbor Freight. I'll guarantee you'll use it again.
Rick, March 2006
Use a 2/4 and jam the blade so you can undo the bolt that holds it on ..I`m lucky to have impacts and a air compressor
bg, March 2006
edit
I can answer this question...
how do i remove blades from my 42inch mtd yard machine?