Change your password. Even if you do block their IP, there would be nothing to stop them going to another computer and trying from that. Also, your acquaitance's IP address may change from logon to logon.
You could use ipfw (IP Firewall) from the terminal to block a specific IP address, but this would not be the ideal solution. If you want to know a bit more, type "man ipfw" from a terminal window to bring up the manual page for the firewall.
If he has access to your Mac WITH your password, I'd change it rather sooner than later. Like mentioned above, he could just go to another computer and work from there, or have a dynamic IP adress from startup to startup