How can I shut off the confirmation prompt when I delete conversations in Messages in OS X?
Solution 1:
After reviewing the preferences as well as defaults, there does not appear to be a way to disable the confirmation message.
Solution 2:
This is more of a workaround. If you enable the "Save history when conversations are closed" option under Preferences->Messages, you won't be prompted when deleting conversations.
Solution 3:
You can hold Option and click Delete or (x).