Here's how I disabled it on my server (v6.11.3):
I added the following in my /boot/config/go file, but you can achieve it in a number of different ways, (e.g. through a user script):
# ignore power button
sed -i 's/#HandlePowerKey=poweroff/HandlePowerKey=ignore/' /etc/elogind/logind.conf
sed -i 's/power) \/sbin\/init 0/power) logger "POWER BUTTON pressed. Ignoring."/' /etc/acpi/acpi_handler.sh
/etc/rc.d/rc.elogind restart
/etc/rc.d/rc.acpid restart
hku2's post in Disable External Power Button No Longer Working Under 6.10 Stable was marked as the answer