Recently, I have seen a problem with my Moodle that seems to be along the same problem as stated in this tracker item.
I have had numerous "bogus" users register on my Moodle. They cannot get into any course because all of my courses require an enrolment key. I do use self-registration, and really don't want to be turning this on and off. I really want to keep this feature enabled.
In Moodle, I can tell Moodle to delete any user not confirmed within (I use) three days. Works great! However, some of these spamming systems seem to be able to read the URL from the confirmation email they (the spamming system) receives, and it completes the registration. They end here (being a registered user), but that's it because they do not have any enrolment key. So my Moodle end up having bogus users not in any course.
I would like to have a similar option in User Accounts|Bulk actions to delete users who are not in any course. As far as I can see, I cannot do this. If I knew SQL better, I could probably create a query against the moodle database, but I am not that good with SQL.
If you think that this should be a separate tracker item, let me know and I can repost it.