Have you tried to track IP addresses? If you can trace them back to your ex-employee you could take legal action on him. If nothing else you will be able to verify your suspicions.
In my humble opinion, I can't see where it could be avast's fault. The best solution is as stated above is to allow one vote per user and no more than one vote per user. Using IP addresses for "one vote" could be a problem as many computers are in homes where every member of the household uses the same machine.
It is not up to avast to "babysit" each user as this would be intrusive and would defeat the anonimity of the user. And there is this "freedom of speech" thing. He can say anything he wants as long as it's not defaming.
You might consider Tech's advice and hire a lawyer.
edit...Since I don't use WebRep I forgot "one vote per IP address.
You might try http://www.reputation.com/
« Last Edit: October 07, 2011, 11:56:06 PM by Para-Noid »
Dell Inspiron, Win7x64 SP1, Comodo Firewall 5.12 w/D+, MalwareBytes Free 2.1--HP Envy Win8.1x64, Private Firewall, MalwareBytes Premium 2.1--Both systems Avast Free v2015.10.2,2218, Super Anti-Spyware, Spyware Blaster, MCShield, Unchecky, Firefox (latest build), Google Chrome (latest build) and, various browser security tools.
When you do something, do it with a purpose and do it on purpose.