A few things you can try..
Check to see if your firewall script is using any log options. iptables -L -n, there'll be "log" rules in there. Those logs are sent to your syslog, most likely. You can either disable the logs from iptables all together by deleting the logging rules, or fix your syslog.conf . That's ultimately how it's printing to stdout.
If that doesn't work, you could add 2> /dev/null to the end of your firewall rules to redirect errors to /dev/null.