Managing users and passwords
Subclipse does not own the information about users and passwords, so there is no way for you to manage it from Subclipse itself. It is controlled via the client interface you have chosen in the preferences. JavaHL shares the same cache location as Subversion command line and other clients that use the Subversion libraries. On Windows this is located in %APPDATA%\Subversion. On *nix it is in ~/.subversion.
The SVNKit stores users and passwords in the Eclipse keyring. It is named .keyring and is in the Eclipse configuration folder by default.
To delete cached password information you can delete the keyring file (in the SVNKit case) or the contents of the auth folder (in the JavaHL case).