In either case: disconnect the spark plug wire and tie it safely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely to prevent accidental starting.
If the nut holding the blade on is just on very tight, use a block of wood to prevent the blade from turning. Use a good quality socket wrench or box-end wrench of the correct size - an adjustable or even open-end wrench may not be enough. The nut usually unscrews counter-clockwise. However, check this out first! A careful inspection of the threads on the end of the crankshaft will reveal the direction. Or, determine the direction of rotation which will be designed to tighten, not loosen the blade. Most, if not all, single blade mowers rotate the blades clockwise as viewed from above which will therefore use a normal right-hand thread nut.
CAUTION: Make sure that if the wrench slips, your flesh will not contact the blade or other sharp sheet metal - liberal use of rags or newspapers is a good idea. Arrange your position and the mower so you are pulling towards you - this is a more stable controllable arrangement.
(From: Graduate student of school of hard knocks.)
"I'd wish I'd read this a few years ago. I used an open-end wrench and it 'flexed' off of the bolt. Needless to say, my next week wasn't a lot of fun with 10 stitches in my hand."
Use some penetrating oil (e.g., liquid wrench or WD40) on the nut and threads if there are signs of rust or corrosion. Allow it to soak in for a few minutes before attempting to remove the nut.
You will prevail. A hammer or other more violent approaches should not be needed.
Once the nut is loose, unscrew it the rest of the way by hand and remove any washers or mounting plate and note their exact position and orientation. The blade and adapter should come off easily. Some penetrating oil (e.g., WD40) may help if it is difficult to remove.
If your adapter/blade doesn't pop off after removing the nut or bolt, it may be mounted using a taper like the flywheel. This is somewhat unusual on a walk-behind lawn mower but might be present on a larger machine like a lawn tractor. A wheel puller is best for dealing with this situation but first see if it isn't just gummed up or rusted in position - try the WD40.
Inspect the key or locking tab for damage. You may have:
If the adapter's tab is broken off or the key is sheared or damaged, then replacement of the entire blade adapter or just the key (depending on your mower's design) will be needed upon reassembly. For now, if you will be sharpening the blade, replace all the hardware in the correct positions (except the blade) and finger tighten the nut so you won't lose anything.
WARNING: Do not install a hard steel key in place of the recommended blade lock key as you will lose the protection that the soft metal provides and the next incident may be the last... See the section:Why soft metal keys must be used.
Once you have reground the blade or obtained a replacement, reassemble in reverse order and then tighten the nut to the proper torque.
(From: Gib Gahan (email@example.com).)
Another way to remove a stubborn blade is to take it to your friendly garage or tire changer and have them put an impact wrench on it. Saves knuckles, tempers, etc. Just don't put the blade back on without a touch of oil or anti-seize compound and of course, don't use an impact wrench!