Dear …
Thank you for your recent proposal offering us solutions for our (name of project/campaign). Regretfully, we must decline your proposal. Though your ideas are very insightful and interesting, we have determined that they are not compatible with our current market conditions.
If you would like to further discuss the points of our decision, please feel free to contact me at any time. I would be happy to discuss this with you.
We do appreciate your effort to find a solution for our needs and we will keep you in mind for future projects.
Sincerely,