Enrolment

G'day!

We're using moodle 1.6.1 and we're doing fine!
It's just that we have some courses that are free to enrol (the student needs at least to login to moodle), and it works like this:

the student has 30 days to do the lessons and the questionnaire. After that, he should be kicked out of the room. We gather the grades every 15 days to prevent losses.

However, there's people there that did the questionnaire months ago, are still listed as students and their tries are still showing.

Weren't they supposed to "vanish" from the students list? And their tries also?

We're working on a basis of 19,346 registered users (also counting the repeated ones), is this size a problem for the DB (it's MySQL) integrity?