You can try any of these three methods (presuming you're referring to the Windows Phone 8.1 version of Viber):
1. Go to Settings > Battery Saver > prevent Viber from running in the background;
2. Launch your Viber app, go to the settings > notifications > disable everything you find in there;
3. Launch your Viber app, go to the settings > privacy > deactivate account. Of course, you would have to re-register your device into Viber after this method.