The contacts that you see in WhatsApp come from your address book. To delete a contact from the WhatsApp Contact list, you will need to delete the contact from your phone's address book.
In order to find the WhatsApp contact in your phone's address book:
Then, refresh your WhatsApp contact list by opening WhatsApp > Contacts tab > Menu Button > Refresh.
If you still see the contact, it is likely that you are seeing the conversation on your Chats list screen. Simply tap and hold on the chat and then select Delete chat.
WhatsApp Support Team