Some shortcuts on Ubuntu 18.04 are not editable in the Keyboard Shortcuts UI. For example the switch workspace to left or to right shortcut CTRL+ALT+LEFT or RIGHT. This shortcut is also useful when you programming in your IDE and you would like to navigate back and forward.
Here a workaround.
# check current settings
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-left
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-right
# disable shortcuts
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "['']"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "['']"