Completely deleting threads off Whatsapp

I use Whatsapp on a regular basis but now I realise that there are some nosy people who have the habit of taking my phone and reading through my chats. I have tried deleting the chat history and the chat backup, however I am wondering if there is any other way to view them and what I can do to prevent this.

It is important that they do not get to read my chats as there are some stuff I deem to be confidential.