Your best bet is going to be contacting Viber, as everyone here helps with Apple products and Viber isn't made by Apple.
I did find some info that might help you though:
If you wish to deactivate your account, go to the “More” tab on your Viber app, and tap on the “Deactivate Account” option. All of your personal information will be automatically deleted.
If you have uninstalled Viber, you will need to reinstall it in order to deactivate your account. To reinstall, please follow the knowledgebase instructions.
If you have lost your device or if it has been stolen, you may reinstall Viber on your new device, using the lost devices phone number. This will once again activate the lost device's number onto your new device. If you wish not to use that number any longer, once activated, you may go to the 'more' option and select deactivate. This will deactivate that number. Then reinstall again using your new number.
If you don’t want to reinstall Viber, don’t worry. Your account will automatically be deleted after 45 days of inactivity.