I am a manager at a company which has struggled the past 2 years with the changing market and due to this has had several budget constraints. Just recently I finally got a budget to redesign one of our web products that actually will be considered that products EOL (end of life) update. After that the product will have no future updates released. The main reason is a pivot in the core of our business to another market segment.
Now that I have a budget, have outsourced the work (all planning and decisions completed for deliverables), and actually have started to execute on the project I received an email from another managers that wants to include their opinions with screenshots of what the update should look like (that boat has sailed already). I worked with one of our business development members for high level input on design but ultimately I am the decision maker.
Our office culture is one that is open to opinions and feedback but in this case I do not want this project to distract our new direction, nor do I want to have it get off the rails with features that simply don't matter. My fear about the latter is that we have an owner who has at times had "knee-jerk" responses to managers feedback that have blown out the budget and scope of projects.
Question: How can you tell coworkers that their input is unnecessary or unneeded, not only because the plan is already being executed on, but to be honest this is not their project to worry about in a polite and constructive way since our culture is one generally open to opinions? In this case it is a manager who also happened to CC about 4 or 5 other managers/personnel on the communication with their opinion. How can I nip this in the bud so this project stays on focus?