If your WhatsApp contact list is huge and increasingly difficult to manage, it may be best to delete some of the contacts that you no longer talk to. WhatsApp uses the your mobile device's phone book to create your contact list. Therefore, deleting a contact from WhatsApp will also remove it from your phone's contacts. This can be done via WhatsApp or directly from your phone book.
Open WhatsApp, and go to the Contacts tab. Then, find and select the contact that you want to delete. Click on their name at the top of the screen, or press Menu (button with 3 vertical dots) and select View contact:
On the Contact Info screen, select Menu > View in address book:
Press Delete (button in the shape of a trash can) > OK.
To double-check that the contact has, in fact, been removed, go to Contacts > Menu > Refresh:
To delete your chat history with this person, go to the Chats tab, press and hold their name for a couple of seconds, and select Delete chat:
N.B. While your contact has been removed from your list, this person can still send you messages. To stop receiving their messages, you must block the person on WhatsApp.
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