How to decline a letter of recommendation request

Hello! I teach 9th grade biology. I received an email from a student I taught last year asking for a letter of recommendation for a summer research program he is applying to.

Some background for this student: He dropped out of honors biology and joined my class after the first marking period. He was very bright and found my class to be too easy, and would goof off in class since he didn’t need to put much effort in to do well academically. He was very loud and disruptive and I called home multiple times for inappropriate classroom behaviors. I caught him cheating on an assignment once, and he did not take ownership for this.

I do not want to write this student a letter of recommendation. The only kind thing I can say about him is that he is academically gifted.

Teachers, have you ever declined writing a letter of recommendation? How do you do it kindly?