How do I unbind Ctrl-Alt-T?

I found this to work for me to disable to keyboard shortcut for running a terminal:

• Press the super key to start a search
• Search for Configuration Editor and select that application
• Drill down the following: apps >> metacity >> global_keybindings
• Scroll down until you see run_command_terminal
• You can remove or type in the word disabled to the right of run_command_terminal
• Press the red 'X' at the top of the Configuration Editor application
• Verify the save by going into: System Settings >> Keyboard >> Shortcuts

Pictures of what it should look like in both the Configuration Editor and System Settings area. Hope this helps!

On current Ubuntu metacity is no longer the default window manager, so the accepted solution no longer works. Instead there's a different setting that you can change. Set the key combination to an empty array.

In a terminal:

gsettings set org.gnome.settings-daemon.plugins.media-keys terminal '[]'

To undo the change again

gsettings reset org.gnome.settings-daemon.plugins.media-keys terminal 

In case this changes again, I found this by looking for all settings that mention "terminal"

gsettings list-recursively | grep terminal

I had this problem because it interferes with an Emacs keybinding to transpose sexps.