Now, on both lists, put a 5 next to your extreme likes an dislikes, a 3 next to your medium-level likes and dislikes and 1 one next to your mild likes and dislikes. Finally, rewrite the lists in order with 5s on top and 1s on the bottom. How likely is it that your areas of dissatisfaction will change. No Say something positive before or after you say no. Make it a genuine compliment.
Bad news is always easy to take if there is some good news along with it.
State your position simply. Avoid getting into the realm of honest critical feedback, especially if you don't plan on seeing this person again. You are not responsible for giving a reason for saying no. You can if you want, but you don't have to. Your no should be clear and effective.