Turn off self-enrollment capabilities for some enrollments

Summary

Is their some way to do this?

Content

We recently started using the Catalog widget to show our enrollments.  We want people to see the all enrollments, but only be able to self-enroll in certain ones.  For most classes, the enrollments are determined ahead of time and are those people get enrolled, but if there are any spaces left in this enrollment, it shows an Enroll button in the widget, and anyone is able to self-enroll.  We need to be able to turn off this functionality for those enrollments.  Since it's not a checkbox on the enrollments page, how do we accomplish this?  Do we manipulate the open/closed dates to be