Viber is currently pushing ads to their desktop applications, to block them you simply block the ads domains in the hosts file for your operating system.

I will show the process for Windows operating system.

First search for notepad in start menu and run as administrator by right clicking it.

Open file menu and click Open, locate the hosts file at C:\Windows\System32\drivers\etc. If you are using any Linux based OS then the hosts file is located at /etc/hosts.

Add the following lines at the end of the hosts file. ads.*

Save and restart Viber or open and close Viber preferences.

Now, enjoy ads free Viber on your Desktop.