There are two options:
1. "Forget" the SSID in question or
2. Remove the certificate being used for the SSID with the following steps:
1. From the Finder, click Go -> Utilities -> KeyChain Access.
2. In the left pane, click on System.
3. Click on the the certificate that you wish to delete:
4. In the menu bar, click on Edit -> Delete. ...
5. In order to complete the change, you will have to provide your password to modify the keychain.