Another alternative:
You can delete the authentication data from the Run-Time Configuration Area: ~/.subversion/auth/ on Unix-like systems or %APPDATA%/Subversion/auth/ on Windows. This is all that TortoiseSVN is doing when you click Settings/Stored Data/Authentication Data.
Yet another alternative:
You can use the --no-auth-cache switch (via the svn.exe command-line client) to specify that you want to be prompted for new credentials on just this one command.
Yet another alternative:
You can edit the config file in the Run-Time Configuration area and set the store-auth-creds option to no. This will prevent the storing of credentials used in any Subversion interactions you perform on the computer. Therefore, you can manually enter new and different credentials on every operation.