Hi Phil
I've got jiggery, fudge, and hocus pocus! 
NB: We're using 2.5, should also work in 2.6 but I don't know about anything past that.
Here's the essence of what I've done in a few places, which does a few different things;
- NULLifies "timecompleted", allowing for a new date
- Deletes any records of Completed course completion criteria; if you don't do this it will just re-trigger a Completion
- Looks for any issued certificate for that person for that course, and deletes it
- Deletes any Quiz attempts ONLY if the quiz is set to limited attempts
This all happens if the user has previously been enrolled but is now inactive; on end of enrolment we suspend the user rather than remove them, as that would remove all of their data for future tracking. So if you re-enrol while still active this step just gets skipped, UNLESS it is a course with shortname starting with "R", in which case every re-enrolment clears the data no matter what (these courses have long enrolment periods and people re-attempt willy-nilly)
$alreadyEnrolled = false;
$previouslyEnrolled = false; //exists but not active, needs data clearing
if (is_enrolled($coursecontext, $user->id, '', true)) {
if (substr($shortname,0,1) == "R") { //already enrolled and still active in a CAMS course, wipe
$previouslyEnrolled = true;
$mtracemsg = $user->username . ' still enrolled in course ' . $shortname . ', clear existing CAMS data';
mtrace($mtracemsg); $adminlog.=$mtracemsg."\r\n";
}
else{$alreadyEnrolled = true;} //already enrolled in other course, leave alone
}
else if (is_enrolled($coursecontext, $user->id, '', false)) { //enrolled but expired, wipe
$previouslyEnrolled = true;
$mtracemsg = $user->username . ' previously enrolled in course ' . $shortname . ', clear existing data';
mtrace($mtracemsg); $adminlog.=$mtracemsg."\r\n";
}
if($previouslyEnrolled){
$enrollinstances = $DB->get_records('enrol', array('courseid' => $courseid));
foreach ($enrollinstances as $enrollinstance) {
$plugin = enrol_get_plugin($enrollinstance->enrol);
$plugin->unenrol_user($enrollinstance, $user->id);
}
$DB->delete_records('course_completion_crit_compl', array('userid'=>$user->id,'course'=>$courseid));
$DB->set_field('course_completions', 'timecompleted', NULL, array('userid'=>$user->id,'course'=>$courseid));
$certid = $DB->get_field('certificate', 'id', array('course'=>$courseid));
mtrace("certid: " . $certid);
$DB->delete_records('certificate_issues', array('userid'=>$user->id,'certificateid'=>$certid));
$quizinstances = $DB->get_records('quiz', array('course' => $courseid));
foreach ($quizinstances as $quizinstance) {
mtrace("quiz id: " . $quizinstance->id . ", attempts limit: " . $quizinstance->attempts);
if($quizinstance->attempts != "0")
{
$DB->delete_records('quiz_attempts', array('userid'=>$user->id,'quiz'=>$quizinstance->id));
$mtracemsg = 'Quiz has limited attempts, deleting previous quiz attempts';
mtrace($mtracemsg); $adminlog.=$mtracemsg."\r\n";
}
}
}
mtrace("alreadyEnrolled: " . $alreadyEnrolled);
Depending on where you are putting this you may need to add various Required code... been a long time since I did it so don't know which is which.
[EDIT] Oh, looks like it must also have a "$coursecontext" already created before this step, you might need to do some digging around to figure that out if you haven't got one defined in your code already