Use File > Exit Directory Opus and then restart Opus.
From a quick look, 7-Zip isn't actually removing its menu items; it's leaving them in the registry and instead deleting some registry entries required to create the COM object that implements the menu items. If the COM object is already loaded into a process when you change the 7-Zip option, then the menu items will continue to work in that process. Restarting the process solves things.
(Waiting a few minutes would probably also solve things, since idle COM objects usually get unloaded after a few minutes.)