Certificates are never deleted because deleting the certificate from the local machine does not delete it from the remote machine to which it was issued. Therefore instead of removing certificates from the local machine, they are revoked.
The CA keeps a database of what certificates were issued to whom, when, and whether or not it is valid. Revoking a certificate marks it INVALID in this database.
To revoke a Host Certificate click the View link next to its Common Name. On the edit screen check the verification box:
Finally click "Revoke Certificate" to complete the process. The certificate will remain in the listing but will appear with lines through it, indicating it is no longer valid.