Comment 1 for bug 646563

I don't know of a built-in way to do this.

When we rip out passwords from the DB, this should be done as a side-effect (see bug 629167 / step 2 of https://dev.launchpad.net/LEP/OpenIdRoadmap). I could mark this as a dupe of that bug, but I'll just point the two at each other instead.