There’s an option within Messages on the Mac to close a conversation you’ve had with somebody, in which case it’ll disappear from the list. Just click the X at the right of the entry in the conversation list. However, the conversation doesn’t actually get deleted. To return it to the list if you remove it by accident, follow these steps:
- Ensure you’re online in Messages with all your accounts — that is, at the bottom of the Messages application window, ensure you select Available.
- Open Finder, or click a blank spot on the desktop, then tap Shift+Cmd+G. In the window that appears, paste in the following: ~/Library/Containers/com.apple.ichat/Data/Library/Messages/Archive
- Your conversations will be listed by date, going back a long time. Just open the relevant folder and double-click the .ichat file relating to the conversation.
- The conversation will open in a chat window, but don’t yet close this. Instead, look at the main messages list in the Messages application window. The conversation you’ve opened will be added to the bottom of the Messages conversation list. Click it to open it there, then close the new conversation window you opened earlier.