"Thank you for your feature request. Submitting thoughtful requests like this takes time, and I appreciate the consideration put in to prepare it.
Unfortunately, the feature request does not align with the goals I've set for this project, and I will not be considering it or any related requests for the foreseeable future. Specifically, I intend to accomplish [your desired outcome of the project] and do not want to reach that goal using the methods you have described.
I sympathise if this is a dealbreaker for you. I have appreciated your patronage up til now, and understand if you need to seek alternative software solutions as a result."
You can follow up with a more "direct" comment if they don't get the message from the above. However this should suffice in most cases.
Unfortunately, the feature request does not align with the goals I've set for this project, and I will not be considering it or any related requests for the foreseeable future. Specifically, I intend to accomplish [your desired outcome of the project] and do not want to reach that goal using the methods you have described.
I sympathise if this is a dealbreaker for you. I have appreciated your patronage up til now, and understand if you need to seek alternative software solutions as a result."
You can follow up with a more "direct" comment if they don't get the message from the above. However this should suffice in most cases.