How can I ensure my students do not send me copied work?

You will require plagiarism detection plugins installed on your site to stop students from sending you copied work. Here are some commonly used plugins - https://moodle.org/plugins/browse.php?list=category&id=35

To get the plugin to work, you will be required to enable, "Enable Plagiarism plugins" feature. This can be enabled in, Site administration > Advanced features.