There are two types of contacts in the application, the ones that are your Phone contacts (contacts of your Phonebook) and Voxofon contacts.
All contacts should appears under "All" section in Voxofon contact list.
If the contact you'd like to delete is your phone contact then you should navigate to your phone contacts list (your phone-book) and delete it from there. It will automatically delete the contact from Voxofon application "All" contacts list too.
If the contact is Voxofon contact you should see it under "Favorites" and "All" sections and you can delete it if you open contact details screen, tap on "Star" icon and confirming contact deletion.