Invalidating users password once authenticated

Hello,

I would like to invalidate user's password as soon as they are authenticated. Invalidate = put some random, hard guessable, value. Does anyone have any idea what would be the most efficient way to achieve this?

Background story: I am distributing password to users after they have been authenticated with some third party. If the password is intercepted I would like to bound its' usage to only one authentication (on top of the small time window that user has to use the password).

Thanks,
Rastko