In trying to remove the blades, the bolt holding them on is very rusted. I
can't turn the bolt without turning the blades as well, so it never loosens.
Any ideas?
Disclaimer:
SAFETY FIRST turning the blade turns the crankshaft which could result
in it kicking forward or back & even starting, at least pull the plug
wire, better yet the plug.

Now I use an impact wrench, but in my primitive days,
Plan A: I'd jam the blade with a chunk of 2 X 4 of other expendable
object, put the biggest box end wrench available on the nut, and hit
the wrench with the largest available hammer.
Plan B: Jam the blade as in plan A, but instead of a hammer, I'd link 2
wrenches together box over open to get a really long lever.
Plan C: if all plan a or b does is round off the bolt, a 3' long pipe
wrench will often win the game.
Plan D: sharpen the blade with a 4" angle grinder right where it is