How to stop the Viber process? I have an HTC Desire S II.

I use Viber only occasionally. To save battery, I want to "kill" it when not in use. The menu has an Exit button that seems to work, but Viber still comes back to life after a while. 

Is there any way to make it stay "dead" without uninstalling it?
This isn't possible, since Viber needs to be running in order to receive calls which are routed via Viber. If you shut down the background process, you can't be reached via Viber. The exit button just means it closes the UI, but keeps listening for calls in the background.