My personal opinion on this is that I agree 100% with dok's post at that link.
Of course, your situation may be a bit different in that you seem to just want to politely reject the interview with the second company since you already accepted the first position. I see no harm in just explaining to the other company that you already accepted another position, but just be sure to thank them for considering you. I don't think you will burn any bridges that way. One thing that's always good to do also is to send the company (or more specifically, the individuals who interviewed you) a thank you letter in writing.