That’s a very vague request. It’s not really clear what you need, so I’m making a guess here…

Assuming you have a list of all courses available, and a list of courses the student has already taken, you could create a new list is the result of a subtract of the list of courses taken from all courses. This should just give you a list of courses that the student hasn’t taken. You can then let the student pick only from this list.

Read more about list operations in the Mendix documentation.