You can disable Virtualmin plugins using System Settings > Features and Plugins page.
If you don’t need services as FTP, PostgreSQL or Webalizer you can also delete those packages using package manager (as longs as they are not a dependency) – not all packages can/should be uninstalled.
I wouldn’t recommend removing Postfix and BIND system packages. However, you can try removing any package you wish, and see if it has any dependencies, if not, most of the time it’s safe to remove it. You must clearly understand what you’re doing, otherwise, just stop and disable the service using systemctl stop/disable service command, and switch it off as a feature from System Settings > Features and Plugins page.