In the SYSTEM > Setup page, select the Startup/Cron tab.
Select the Post Boot script from the pull-down menu
Edit the Post Boot script:
>>> SNIP <<<
# Startup Script
# Disable the COM1 serial port console
[ -f /etc/inittab.original ] || cp /etc/inittab{,.original}
/etc/inittab
sed /agetty.*localman.*ttyS0$/s/^/#/
telinit q
>>> SNIP <<<
Select Status: Enabled
Select Save
Reboot.