Open the Messages app on your Mac
Select the conversation containing the iMessage you want to delete
Right-click the message bubble
Click Delete
Confirm the deletion if prompted
To delete an entire conversation, right-click the conversation in the sidebar
Click Delete Conversation
Confirm the deletion if prompted
To delete multiple messages, hold Command and click each message
Right-click one of the selected messages
Click Delete
To delete all messages in Messages, open Messages
Click Messages in the menu bar
Click Settings
Click General
Adjust message retention settings if needed
Delete conversations manually for full removal