How to reset Messages application settings in Mountain Lion?

Solution 1:

Messages.app uses many files in various locations for storing information.
To completely start from scratch, all these files must be deleted.

1. Delete the preference files (there are many of these):

   find ~/Library/Preferences -name "*iChat*" -delete
   

2. Delete caches:

   rm -rf ~/Library/Caches/com.apple.Messages
   

3. Delete chat database:

   rm -rf ~/Library/Messages
   

^{Remember to take a backup before deleting files.}

Solution 2:

Mac OS X 10.8.2 fixed the problems.

I started the app after the upgrade and the double account was still there. I opened the preferences, the app crashed and, when I restarted it, the problems were gone.

Solution 3:

To reset Messages app to default configuration you must delete your user preferences for the application. From the finder toolbar select Go > Go To Folder and enter ~/Library/Preferences/. In this folder you need to delete the files that match com.apple.iChat.plist, or any sub-setting files like com.apple.iChat.Jabber.plist which will exist for each type of account you have existing your Messages app.