You are not telling us your distro, so it is more difficult to help you.
If your distro has the file /etc/inittab then find the entry that refers to whatever is respawning the getty process for ttyx (your serial port) is, and comment it out.
Then you'll either have to reboot, or kill the getty process for the serial port you do not want a login manager sitting on. You can't just kill it without first editing /etc/inittab because otherwise it'll just be respawned.
If you are using a recent ubuntu, take a look in /etc/event.d/ttyx
Last edited by tredegar; 05-05-2009 at 11:03 AM.