Hi,
You can prevent users from logging in to your site by setting their authentication method to 'No login'. Alternatively, Moodle 2.2.1 includes an option to suspend a user's account. Both settings are found on the edit profile page, though for 9,000 students you'd be wanting to make the change directly in the database.
Another option would be to just prevent users from accessing their courses, whilst still allowing them to log in. Please shout if you'd like further details of this option.