After flattening the lock-washer, I used a 1/2" electric impact gun to spin off the nut. I did not need to lock-off the tire.

However, the sprocket itself was quite tenacious! I needed to use a small gear puller to remove the sprocket from the shaft.

 

That's not good. Make sure that the splines are clean and free of dings-it should be a slip fit.

 

I like the wood chisel idea, screwdriver and hammer kinda tears up the nut. Once away enough from nut hammering the socket to finish flattening is a great idea. I have a two foot breaker bar, kind of a long non ratcheting socket wrench. I have wife stand on rear brake lever while I stand on breaker bar and bounce...

 

I use a small blade screwdriver to start bending the washer, then a really big tip blade screwdriver to bend the washer flat.

I also hold the brake and use a long breaker bar. Always torque it back on, probably will slather on some anti-seize next time too... you guys are scaring me.

I really liked Honda's method of using a splined retainer in a groove that held by using a 1/8 turn that would line up two bolt holes to bolt on the retainer in locked position. Clean and quick.

 

Or, KTM only uses a snap ring with no bolt. Easy peasy.....

 

My KDX is this way. Easy but does cause more spline wear. Ktm has now gone away from the clip.