Apple: Deleting The Keychain
1. Open the hidden Library folder. To do this, click the Go menu in the Apple Finder. While the Go menu is displayed, hold down the Option(Alt) key to reveal the Library menu option. While still holding the Option(Alt) key, click Library. The Library window will appear.
2. Delete the user’s login.keychain file (or move it to a different location). This can be found in the Keychains folder, which is located in the User’s Library folder.
3. Relaunch the afflicted app. On relaunch you may be presented with the ‘Keychain Not Found’ dialog box which states that ‘A Keychain cannot be found to store “Safari” [or other app]‘. If this happens, click ‘Reset To Defaults’.
4. Confirm the keychain reset by clicking "Yes".
5. You will be prompted to enter the user's login password to create the new keychain. Enter the password and click "OK".
6. Relaunch the afflicted app.
Source: http://www.umkc.edu/is/support/services/macintosh/keychain.asp