Open WhatsApp
Go to the chat with the number you want to block
Tap the contact name or number at the top
Scroll down and tap Block
Tap Block again to confirm
Open WhatsApp
Tap Settings
Tap Privacy
Tap Blocked contacts
Tap Add
Select the contact or number to block
On Android, open the chat
Tap the three dots in the top right
Tap More
Tap Block
Tap Block to confirm
On iPhone, open the chat
Tap the contact name at the top
Scroll down and tap Block Contact
Tap Block Contact to confirm