Dear [Hiring Manager],
I am writing to inform you that I will be unable to attend the interview scheduled for May 16th, as I have accepted a job offer from another organization last April 1.
I apologize for any inconvenience this may cause, and I want to E×ρréšš my appreciation for your interest in my application. It was a difficult decision for me to make, but I ultimately had to accept the other offer due to the immediate start date and the opportunity it presented.
Once again, I apologize for any inconvenience and thank you for considering me for the position. Please let me know if there is anything else I can do to assist you.
Sincerely,
[Your Name]