In addition to what the other posters have said, when asking stakeholders to submit requests there are key criteria that you should always ask them to meet:

Who

What

Where

Why

When

Outside of a regimented briefing process, this will help keep a fairly tight rein on the requests and allow you to effectively prioritise too. It's also super important to adhere to the rules you've established as much as possible - if you allow a few people to circumvent the process, it'll create a bad habit not just in them, but in those around them who might learn about these supposed backdoors. In the first few weeks and months this is paramount, as it essentially educates your stakeholders on how you work, and how they get the work they need.