This menu is called the client-menu. Unfortunately there's no way to change the entries aside from modifiyng the source code.
However, if I understood you correctly you want to remove all menu entries aside from the ones pertaining to the window layer. In that case you can assign a shortcut to the client-layer-menu
.
Open your openbox rc.xml
(usually under $HOME/.config/openbox/rc.xml
, might be called lxde-rc.xml
or lubuntu-rc.xml
depending on your DE). Find the following line:
<keybind key="A-space"><action name="ShowMenu"><menu>client-menu</menu></action></keybind>
and replace it with:
<keybind key="A-Space"><action name="ShowMenu"><menu>client-layer-menu</menu></action></keybind>
As for the window close option you lost by replacing the menus: You can always use the ALT + F4 shortcut.