Hi, Matthew

This could be achieved by overriding the permission for the required role e.g. Learner, at the course level, although, by default this capability is normally not set for any of the default roles.

For manual enrolments, there is a capability enrol/manual:unenrolself that you can search for and prevent at the course level. This would disable the ability for Learners to un-enrol themselves from a course.

1. Within the required course, go to: Course administration > Users > Permissions
2. From the drop-down, select the required role to override
3. Search for the capability enrol/manual:unenrolself (this is specific to manual enrolments)
4. Select the Prevent radio button from the Permissions list

There is more information on the Moodle docs pages here - http://docs.moodle.org/22/en/Capabilities/enrol/manual:unenrolself

Cheers

Tom