In GNOME 3.8 you can switch workspace by using the Super + Page Up or Super + Page Down. Unfortunately you can do the same with Ctrl + Alt + Arrow Up or Ctrl + Alt + Arrow Down.  The last one conflicts with a neat shortcut in Eclipse to duplicate lines.

Under Settings > Keyboard you will find the hotkey to switch workspaces. The Ctrl + Alt Arrow Up/Down is always bound and is not shown.

Check which keys are bound here:

gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-up
['<Super>Page_Up', '<Control><Alt>Up']

gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-down
['<Super>Page_Down', '<Control><Alt>Down']

You can clearly see that both commands have 2 hotkeys assigned.

To remove the Ctrl + Alt variant run this:

gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-down '["<Super>Page_Down"]'
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-up '["<Super>Page_Up"]'

The hotkeys Super + Page Up and Super + Page Down are retained but Ctrl + Alt + Arrow Up and Ctrl + Alt + Arrow Down are removed.