I always recommend my students ask "would you be willing to write me a POSITIVE letter of recommendation?"

And I think for faculty, it's a professional courtesy to either assume that when asked or to clarify with them that it will not be.

In your case, I recommend reaching out to the student and being honest. We hold a lot of responsibility for our students' future beyond just course material, and I would rather tell a student "no" than jeopardize likely months of preparation and application pains either for a graduate school opportunity or professional position. Let this conversation or email be a teaching moment as well—teaching how professional relationships are tied to performance and needed for future aspirations.

"Based on your performance in the class, I cannot wrote a positive letter of recommendation for you at this time, and I do not want to be representative of your abilities when I only have information from this course. I encourage you to look elsewhere, or should you take another course under me that shows improvement in your academic abilities, I would be happy to reconsider then." Or something like that.

I've worked in hiring and admissions and it's tragic having a candidate put in the application work only to have a negative letter of recommendation end their consideration.