Google is your friend here. You want some rate limiting. There seems to be a good one liner here, but I would use -I instead of -A. Also, for a custom firewall rule use $IPTABLES and not "iptables". Use "iptables" at the command line. Check the rule does not return any errors at the command line first.

And here is a much bigger rule set with tweaks. If I were applying them, I would test at the command line then add to /etc/clearos/firewall/local, again using $IPTABLES rather than iptables.

Thx! All works. Very nice.