To get into your 'Student Non-Activity' reports, hover over the 'Marking & Reports' heading at the top of the screen and click on the 'Student Non-Activity' button which will be displayed underneath the 'Reports' section
You will then be taken to a page that looks like this. Here you can view and manage all users that have registered but have not started any courses
To De-activate a single user you can click on the red button to the left of their details.
If you want to De-activate more than one user, you can click each box on the right of the users details and then go to the drop down box on the right hand side that reads 'Select Mass Action', click on this and chose the 'De-activate' option.