I attempted to remove all StartCom certs from the OS X keychain, apparently you can't do it even as root. You have to boot into the recovery partition first.
I can't remember whether it was MacOS or Windows, but at least one OS will sometimes re-install a 'standard' CA certificate if you delete it. Setting 'never trust' is the most reliable way to kick out the CA as it ensures that it stays out.
http://superuser.com/questions/1070664/security-seckeychaini...
Edit: Don't delete them, instead right click each certificate, select "Get Info", Expand the "Trust" panel, and set it to "Never Trust"