MultiCommander v14.2 is released!
In the Configuration > Manage Plugin and Extensions -> FileSystem Plugins.Select FS-7Zip then click on Optionthen double click on 7z in the list and click on remove. And close and save changes and restart MC. Now .7z file will not be handled as virtual filesystem and if you click on them they will be launch, and if 7Zip is installed it should open it that.
Disable by checkbox might not work for built in extensions..