Viber is available as a .deb package and also as tarball. If you installed viber through the .deb package, then you can easily uninstall it using sudo apt-get remove viber

But instead, if you installed through the tarball, then you must have extracted the viber files into a single folder such as /opt/Viber. Just delete that folder and any .desktop files (shortcuts) that you created for it.

In addition, Viber user configurations are stored in ~/.ViberPC. You can choose to delete that as well. Note that if you do this, the next time you install viber, you would have to make any customization you made this time , again.