Viber uses ports 5242, 4244, 5243, and 9785 along with 80, and 443 for http and ssl (obviously) so what you can do is go to "Firewall Objects", "Services", add a service and type all 4 ports separated by commas. Name it viberports or something. Now you can block traffic on those ports.
Also whatever you do do not block 80 and 443 for obvious reasons
Edit 2: make sure to use UDP and TCP as viber uses udp for video and tcp for text, but you probably already guessed that.