You know, you had to see what that module did. Its part of the basic install, and you may need it again, so deleting just isn’t an option. So move it back to “Un-used Modules” should be easy! Not so much.
In the file manager or your favorite terminal go find the folder /etc/webmin or /usr/local/etc/webmin. Find the file named “installed.cache”. Pop it open and change that modules line ending from an “=1” to an “=0”, save, logout and login to Webmin, annoying module gone. (hidden anyway)
Disabling is more of a permanent solution. For instance, Fail2Ban was installed, fiddled with a bit, and removed. So now if the module is hidden from me, I won’t remember hiding it. So in 5 or 6 months I might want to play with it again (new version, something) and can’t find it. Ugh.
Stick it back in Un-Used, it can happily sit there until I want to experiment again, unseen and not annoying me.
Webmin module checks for a service config directory or config file to understand if the service is installed or not. In case of fail2ban if you’re no longer using it (uninstalled it) removing its config directory (i.e. /etc/fail2ban) and refreshing unused modules should force fail2ban Webmin module to be removed from active Networking category.