How to block subscribers

On one of our blogs, we have unticked “anyone can subscribe”.

We don’t want public members – only those people we want to post, who have been manually added as users, through the admin menu.

We have “subscriber” as the default membership level.

I just noticed about 5 usernames I haven’t seen before – they have added themselves as subscribers.

We don’t allow self-registration.
If I go directly to /wp-register.php, it tells me “user registration is currently not allowed”.

The five users were added at random times between May and September this year, based on the timestamp on the database.

Any ideas how this could have happened?

We have links to feedburner for subscription purposes.