How to decline an interview meeting?

I have been looking for work for the past 6 months and was just recently offered a full-time position to work at a newspaper company and I accepted. However, before I was offered the position, I made an appointment next week to meet up with the creative director of a design company. He was going to review my portfolio and see if I would be a good addition to their firm and work as a freelancer. Since I already accepted another position, how do I decline the meeting with the creative director? What exactly do I say so that I can remain in good terms with him and the firm so that maybe in the future, if they do look for a full-timer, I could re-apply?