By default the /Library/Preferences/com.apple.pcastserverd.plist allows basic, digest and Kerberos authentication. Attempts to authenticate will be made in the reverse order, respectively. This is pulled from the http_auth_type array, which you can see using the following command:
serveradmin settings pcast
You can then remove an entry and edit existing entries to change the supported mechanisms using serveradmin if you cannot stop the Podcast Producer service. If you can stop the service then the easiest way to edit the authentication mechanisms is to edit /Library/Preferences/com.apple.pcastserverd.plist directly. To do so, locate the http_auth_type key as you see it here:
Here, remove each string that you no longer wish to support. Removing all except Kerberos will provide support for only Kerberos as an authentication mechanism.