The notification email you sent me was for a Trackback (note that it contains "Tracked on ..." which is only added for Trackbacks and Pingbacks).
Trackbacks are, by definition, always anonymous. So your only option would be to disable them entirely.
However, to my knowledge, Geeklog does a pretty good job at catching Trackback spam. I usually set $_CONF['check_trackback_link'] = 5; and haven't seen a single Trackback spam making it through since.
The spammers are still trying it of course, which is why you see the entries in the Spam-X log.
bye, Dirk