This page describes how to administer SSL certificates in MacOS X 10.5. Other versions of Mac OS-X may be similar. Instructions for other environments are also available.
Sometimes it is necessary to delete a certificate that you have already accepted. For example, if you have previously accepted a self-signed certificate that has expired, you may need to delete it from your Mac KeyChain before you can accept its replacement.
Steps for deleting a certificate:
From the Finder, click Go -> Utilities -> KeyChain Access
In the left pane, click on System.
Click on the the certificate that you wish to delete:
In the menu bar, click on Edit -> Delete. You will be prompted for confirmation.
In order to complete the change, you will have to provide your password to modify the keychain. This is likely your login password:
After a brief wait, you should no longer see the certificate in the certificate list.