Administration hacks
From MoodleDocs
Preventing Welcome to the course emails
You can stop moodle sending enrolment emails by changing two lines in the file /enrol/enrol.class.php
The line (occuring twice in the file!) reads:
email_to_user($USER, $teacher, $subject, $message);
It must be changed to:
// email_to_user($USER, $teacher, $subject, $message);
That way, moodle ignores the e-mail-command until you change back to the original line. The setting works for all courses at the same time. It does not affect other e-mails being sent (e.g. the login confirmation).