From android 6.0 and up you can go to Settings -> Type "Permissions" into the search -> Select "App Permissions" -> Then again select "App permissions" (you may not need to do this twice) -> Press Camera -> Find WhatsApp and swipe the button.
Caveat: This will also disable you from opening the camera in whatsapp too i.e so if you press the camera button to take a picture in a chat this won't work. you would now need to take the image first from the phone's camera app then find it using the whatsapp gallery (or in-phone gallery) to share it.
Below 6.0 there is no way.
WhatsApp don't have a setting to disable video call so these are your only work arounds.
If you don't use the front facing camera at all I guess you could cover it with a sticker/post it.