I'm using the script mentionned earlier and it works almost perfectly...
I invoke it from /jffs/scripts/firewall-start. I make the request per ip instead of a scope because its easier for me to manage. The thing is that it works for a few hours and then my machine (security.maison.lan) is not able anymore to send emails until I run :
/jffs/scripts/block-ip-ranges.sh "40" delete