You can prevent a contact from sending you messages by adding their phone number to your Blocked list. To learn more about the effects of blocking a contact, see this article on blocking contacts in WhatsApp.
On iPhone, there are two ways to block a contact:
- Use the [Block] button which is displayed when you get a message from somebody who is not in your address book:
- Go to WhatsApp > Settings > Chat Settings > Blocked:
Tap on the [+] button to add contacts from your address book to your block list.
Be sure to tap [Done] when you are finished adding contacts.
To unblock a contact, open the Blocked list with the above steps, then tap [Edit], then tap to the left of the contact, then tap the [Delete] button, and then [Done].