Student creativity when I need to grade their fulfillment of the expectations

I am not a teacher to automatically stifle creativity in order to get all my students to think my way. But I have a student this semester who wants to challenge every assignment and I'm not sure how to deal with them. I think part of the problem is that this is a senior in a 100 level class.Their first paper was an ahistorical metaphor and now their second paper outline wants to primarily use outside sources, when the assignment was to deal with the material presented in class (i.e. not do a research project, which this department doesn't assign to 100 level classes).

How do you assess the line between creativity and fulfillment of the assignment? Is fulfillment a baseline, on which creativity is built, or should creativity be honored no matter what form it comes in? Where is the line between requiring the same expectations of every student and responding to the particular needs of individual students? We say we want students who are concerned more with the learning than with the grade, but what does that actually look like? Will they take such responsibility for their learning that it becomes something other than what you intended for the course (because it is what they want to learn)? How do you provide flexibility while maintaining the cohesion of the course? How do you grade someone who is so far off the grading rubric?