Not that I'm an expert by any stretch, but it sounds as if you are
seriously over tightening your arbor nut. For an alternate opinion
check out the Q&A section of the Winter 2004/2005 FWW (no affiliation
with them, just happen to see the article this week). It suggests hand
holding the blade to tighten and loosen the blade by whacking the wrench
with a small stick. This would mean the nut is much looser than yours
is now. I'm sure there are other opinions out there.

Tom Holman wrote:

Trying to remove a plywood blade, and when I lodge a piece of hard maple
against it, it is not enough to prevent blade from turning and nut on arbor
will not move. I am trying to move it counterclockwise. Anyone have any
tricks? Help!