Thank You for Interview, Not Interested: I Have Accepted another Offer
[Company Name & Address]
It was really great to meet you on [date]. I have received the offer letter through mail and I am really thankful to you for selecting me.
However, I had already accepted a job offer from [Other Company name] before I got your offer.
But, I am really grateful for considering my candidature with your company.
My best wishes for your business success.