Unfortunately, there is no built in function in Acrobat or Reader (7, 8 or 9) to do this. Acrobat X now has has the ability to clear cached credentials.
If you absolutely need to get rid of cached credentials, you can try deleting the files located in C:\Documents and Settings\userprofile\Application Data\Adobe\Acrobat\9.0\Security.
Please note that you should make a backup of the files before trying this. Also, you will lose any principal keys (used for offline access) that were previously downloaded. Acrobat\Reader will prompt you to re-download them for documents where the user has the offline permission.
Regards
Steve