Someone could create a mod like RTB called the GUI Safe.
People could submit their GUI's to the GUI Safe team and they could approve of it like the RTB team does with their add-ons.
Then there would be some kind of check when joining a server, if the GUI has been approved by the GUI Safe team it gets direct downloaded, but if it hasn't been approved it gives you a warning that GUI Safe has not approved it, and it may not be safe, and you get disconnected from the server. Basic GUIs are fine because they can be downloaded safely, but for more complex GUIs, someone should make a "GUI Safe" team/mod of some sort. This would pretty much fix the whole problem.. right?