Sprocket failure

This is either misleading or dangerous.

You don't necessarily need a chainwhip (rotafix has been mentioned before) but you do need a lockring tool* (the pedros trixie will be fine although I have the shimano dura ace and think that it is better (more comfortable and more leverage)).#

*You can get away with using a hammer and chisel, apparently, but I think this is a silly idea.

I've used a hammer and chisel on an old-style bb. It was a silly idea but it did do the trick.