WhatsApp automatically inserts a "You blocked this contact" or "You unblocked this contact" message into the thread. These cannot be deleted individually. To remove them without losing your entire conversation, use this community-verified workaround :
Advanced Workaround: Remove "Block/Unblock" Notifications Only how to delete unblock messages in whatsapp
Note: The "Delete for Everyone" option is usually only available for a limited time (roughly 2 days) after a message is sent. How to Clear the Entire Chat History WhatsApp automatically inserts a "You blocked this contact"
Maya decided to delete the entire “Old Project Team” chat. Then she messaged the admin: “Hey, unblocked you. New chat, fresh start!” They laughed about the old spam, and everything was clean again. unblocked you. New chat