"Thank you for the proposal but unfortunately it does not fit in the scope and vision I have for the application, and I rather spend time on areas that I believ

"Thank you for the proposal but unfortunately it does not fit in the scope and vision I have for the application, and I rather spend time on areas that I believe in. However if this is very important to you, I can consider implementing it for a fair compensation for my time." Something like that. And ensure that contract doesn't include lifetime support/maintenance... :)