You can easily block unwanted traffic using IPTABLES. However, if your using csf/lfd from configserver you can easily block specific traffic.
I whipped up a BFD rule to try and defend against sip attacks. This watches for failed logins in the /var/log/asterisk/full log and then blocks them. I set the TRIG level low so you might need to change this to something you are more comfortable with. Drop this file in /usr/local/bfd/rules/ Name the file …
I have created a small guide to help users secure their trixbox CE installs. This guide is in PDF format and should be readable on just about any system. This is by no means a definitive guide, but should be used as s way to help users. securing_trixbox_ce_ver1