You can prevent a contact from sending you messages or calling you via WhatsApp by adding their phone number to your Blocked list. To learn more about the effects of blocking a contact, see this article.

On iPhone, there are a few ways to block a contact:

• To block a contact that is not stored in your phone's address book, go to the chat for that person, tap on the number for that person at the top of the chat, and then tap Block Contact.

• To block a contact that is stored in your phone's address book, go to WhatsApp > Settings > Account > Privacy > Blocked, then tap the Add New button.

• When you receive an initial message from a contact that is not stored in your phone, and you want to block this contact, tap the Block button. If this message is spam, you will also have the option to Report Spam, then tap Report and Block.

To unblock a contact, open the Blocked list with the above steps, slide your finger to the left on the contact you wish to unblock, then tap the Unblock button.

Learn how to block or unblock a contact on: Android | Windows Phone | Nokia S40 | BlackBerry | Nokia S60 | BlackBerry 10

Cheers,
WhatsApp Support Team