Viber 4.2 on Android lets you block any number or contact 

Viber has released a new update for the Android version of the app. v4.2 brings with it some useful new features, most importantly call blocking. You can now specify any number or contact you don’t wish to receive calls from and it will be blocked for you.

Other changes include ability to send up to 10 images at the same time, ability to customize your messages and call sounds, new media gallery within conversations and other improvements and bug fixes. 

The latest update is now on the Play Store