I found out how to use Viber when, I want to use it and disable it when I have no need of it.
May be it would be useful for someone.
So take a rooted phone (I have a rooted xperia ray).
Install "BusyBox Free" and "Autorun Manager", run autorun manager in advanced mode, find Viber and disable KeepAliveReceiver, IncomingSmsRevceiver, C2DMreceiver and AutoLauncer.
Now when I exit from Viber, it would not start by himself and when I start it manually it works properly.
I beleive it is very easy for developers to make "autostart toggle" as an option.