How do I prevent students from enrolling themselves in a course?

If you are a teacher in a course:

1. Go to Settings > Course administration > Users > Enrolment methods.
2. Disable (close the eye) of the self-enrolment option.

If you are site admin:

1. Go to Settings > Site administration > Plugins > Enrolments > Manage enrol plugins.
2. Disable (close the eye) of the self enrolment plugin.