This task seems easy to accomplish in Moodle 1.9+...
- Click on the Assign Roles link in the Administration menu. This is usually found on the left hand side.
- Click on Student in the list of roles.
- Select students you wish to unenrol from the left hand column. You can select all students by clicking on the first in the list then holding down Shift and clicking on the last, or you can select individuals by holding down Ctrl and clicking.
- When you have selected all the required students click on the centrally located Remove button to unenrol them.
... but not so in Moodle 2.0, where unenrolling a large number of users from a Moodle course will need the Administrator/Teacher to click on the 'x' in the Enrolment Conditions box next to users' names several times.
Or am I missing something?!