One annoying thing in Mountain Lion’s Messages.app application is that the shortcut command + delete is mapped to “Close Conversation” instead of deleting the current line of text. This fix to this is very simple.

We are going to change the shortcut for the “Close Conversation…” action to something that’s not command + delete.

1. Choose System Preferences from the Apple menu.
2. Choose Keyboard from the View menu.
3. At the top, choose the Keyboard Shortcuts tab.
4. In the left box, click Application Shortcuts.
5. At the bottom of the right box, click the + sign.
6. Fill in the form as shown below in the image.
7. Click Add.
8. Done! Now when you go into messages, you’ll have the usual command + delete behavior.

Note: The keyboard shortcut given above is shift + command + D