"How do you stop it short of putting a block on all incoming mail?"
Don't use "Lookout!" Get Mozilla...
http://www.mozilla.org Other mail readers aren't compatible with the worm. Readers like Mozilla just stare stupidly at the worm without a clue as to how to run it. No run, no worm, no worries. I'm sure that other mail readers are just as incompatible with these MS worms. I happen to like Mozilla for it's anti-spam feature. It can be taught what is and isn't spam. It then moves spam off to a junk folder and deletes it after X days. (This feature has caught roughly 300 spam messages on our account over the past 7 days!)
Of course, a more radical approach is to stop using that one particularly worm-compatible operating system from the "Land of rain and espresso".
: )