Hi Drew. Are you sure you don't have your users assigned site wide via site admin>users>permissions >assign system roles?Because if this is the case then they will appear in the gradebook even though they haven't enrolled in the course. Equally, is your default role set to student and not authenticated user? Because if this is the case it might also cause this behaviour.
Gradebook
Gradebook "Student" list does not match student roles list in 1.7
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