@KendallCNumber Hello - I read over the admin guides on authentication to double check but to confirm, I do not see the ability to exclude email domains nor IP address ranges from being able to make use of a direct enroll link.
https://community.canvaslms.com/t5/Admin-Guide/tkb-p/admin#Authentication
By the way, a few years back when we wanted a custom solution that limited students so they could register for a course using a specific assigned email address domain (for example, .gov) we created the solution ourselves using Cold Fusion forms.
Edit: as of this week we rolled out Catalog and now have that for a course registration solution.