Shoutbox

If you don't like the idea of patching, StuffPlug-NG is capable of removing the contact list adbanner and (in build 777) the text ad in convo windows. The next version will also work for build 813.
OK, it won't remove all the buttons, but you never know what new features could be added, or new plugins released.

As for the Plus icon in the convo window: that's there for quick and easy access, and I think is actually a good UI improvement.