Pull the spark plug and feed a few inches of small rope into the cylinder - if you have some spare starter pull cord that'll do nicely. This keeps the piston from being able to come to the top all the way. And no, giving the wrench a good whack shouldn't hurt anything - just be reasonable (no 5 lb mini-sledge necessary here!). Keep in mind that the clutch nut is reverse-threaded.
I use a portable 18V cordless impact screwgun to remove clutches. The only ones it sometimes can't pop are the 395 & 090, and I have yet to need to block a piston to keep the crank from turning.
