Viber is using the contacts from your mobile device. When you install it, your contacts will be automatically synced. There is no way to delete a contact by using the PC application. This can only be done from a mobile platform. To delete a contact, open Viber (on your mobile device), go to Contact list, long tap the one you want to remove and select Delete. When you will connect next time to Viber on your PC, the deleted person will be removed.