How to say "No" to 1001 stakeholders

As a PO, you have the "keys of the kingdom": You decide what the development team will work on and with what priority. But with great power comes... a big group of people who tell you what to do with that power. It's only natural that the great product you're building is attracting a lot of attention from a lot of people, who all have many ideas about how to make the product better. There's a lot of these people, only one of you, and only a few developers who can actually turn these ideas into reality. So you end up saying "no" to the vast majority of these requests... and probably not making a lot of friends in the process. Some of these people may be your superiors, or your most important clients, or the colleague you go to lunch with every day.

So how do you manage, prioritize and triage these requests? How do you say "no, we won't implement that" for the tenth time to your over-enthusiastic colleague? How do you say "no, that's not on our roadmap" to a paying client? How do you say "no" to the developers who were used to doing whatever they wanted before you were hired as PO? How do you say "no" to the sales representative who already promised a new feature to clients? How do you tell the CEO "no, that is not realistic in our lifetime"?

Based on a concrete case study, we will look for tips, tricks and solutions to help all of us deal with this situation, which is all-too-common in most POs' lives.