I'm an official representative from Viber. I'm here to help :)
Deleting Viber, as opposed to deactivating, makes it impossible for us (technically) to know whether or not that user intends to reinstall Viber later, or plans to stop using it altogether. This is why there is a wait of 45 days during which if we see no activity on the account, it is automatically deleted, and only then you will stop appearing as a Viber user to your contacts.
The instant way to do it, however, is to deactivate Viber instead of uninstalling it. This is done through the "More" tab inside Viber.
If there are any more questions, don't hesitate to let me know :)