To stop/start Open Directory:
sudo launchctl unload /System/Library/LaunchDaemons/org.openldap.slapd.plist
sudo launchctl load /System/Library/LaunchDaemons/org.openldap.slapd.plist
To stop/start Kerberos:
sudo launchctl unload /System/Library/LaunchDaemons/edu.mit.Kerberos.kadmind.plist
sudo launchctl unload /System/Library/LaunchDaemons/edu.mit.Kerberos.krb5kdc.plist
sudo launchctl load /System/Library/LaunchDaemons/edu.mit.Kerberos.kadmind.plist
sudo launchctl load /System/Library/LaunchDaemons/edu.mit.Kerberos.krb5kdc.plist
Note these are 10.6.x specific commands. They're slightly different for versions above and below.