Say "thank you for the opportunity, but on reflection, I have decided that the information I gathered from the interview made me realise that this isn't the route I want to take in my career."
Someone recently said to me that having an interview is hugely good experience (and I agree), and if you get offered the job, you don't have to take it.
Tell them quickly though, so they can offer it to the next best person - there is nothing worse that telling the unsuccessful people, then having to backtrack because the person offered it, declined. That happened to a colleague, she was the second chance person, having been told she hadn't got the job, and felt very "second best".