The question How to diplomatically reject someone's project due to their attitude was just closed for "asking advice on what to do." Honestly, I have no idea how this applies at all. The OP is not asking us whether they should reject the proposal or not, they are asking how to professionally communicate this proposal rejection. These types of questions are at the core of what is on-topic at The Workplace. From our Help Center (emphasis mine):
Leadership in the workplace (motivating people, encouraging people, making decisions, holding hard conversations, intervening in unproductive situations, asking for and giving help, etc.)
This question should be reopened.