Shoutbox

If disabling logs from Plus > Preferences > Logging > uncheck "Automatically log all my chats by default" doesn't work (which by the way is something really weird, do you have any plugins, add-on or something else that could be interfering?), try disabling logging from the registry:
Close Messenger.
Go to Start > Run, type "regedit" and press OK.
In the tree on the left, locate and select this key:
HKEY_CURRENT_USER\Software\Patchou\MsgPlus2\<your e-mail>\Preferences
Now, on the list on the right, locate and double click "EnableLogging", set its value to 0 and click OK. Restart Messenger.