Ubuntu 18.04 - Is there a way to disable the alt+½(the button right above tab) shortcut

You can disable the key by changing a setting. Either use dconf-editor, which is not installed by default, or execute a terminal command.

Using dconf-editor:

Navigate to /org/gnome/desktop/wm/keybindings/switch-group. The default setting is ['<Super>Above_Tab', '<Alt>Above_Tab']. Delete the <Alt>Above_Tab key so it becomes ['<Super>Above_Tab'].

Using the terminal:

Execute the command

gsettings set org.gnome.desktop.wm.keybindings switch-group "['<Super>Above_Tab']"

You can always reset a setting to the factory default:

gsettings reset org.gnome.desktop.wm.keybindings switch-group