How to stop Viber running without uninstalling it?
This is an official rep. of Viber.

Due to the C2DM feature, you are always available on Viber, there is not way to  "log out" and "log in".
Please note that if you "exit" Viber, it will open again only if you get a msg or call.

Let us know if you have any additional questions.

Thanks!
Viber Team