I haven't found a need to ban access to Geeklog yet so I don't know if through Geeklog it's possible to ban IPs.
You can use a firewall unless of course you have other services running on your computer you would like them to access.
A good free firewall which should allow you to block IPs is Kerio personal firewall at http://www.kerio.com---Victor B. Gonzalez
Victor B. Gonzalez -
http://aeonserv.com