Configuring preferences and options in Outlook.com
I had the same issue and after 2 "chats" with Microsoft, I finally got rid of that stupid banner and Copilot in my email. I also uninstalled Copilot on our PC's and I turned off the "Connected Experiences" from WORD, EXCEL, POWERPOINT, etc. (you have to do them individually), I don't want or need AI, especially for email, I think I'm capable to read them on my own.
Check your subscriptions under your MS account and if you're on Microsoft 365 Family (I'm guessing personal too??) that's what's causing the banner in Outlook.com email. Luckily my renewal was just 2 weeks ago, so I was able to have the nice person at Microsoft cancel that subscription, get a refund, and then sign up for Microsoft 365 Family Classic. The Classic version doesn't have all the unwanted and intrusive AI/Copilot junk and the banners are no longer displayed in my Outlook.com emails. If your current subscription is more than 30 days from the renewal date, you can still change to Classic, but you won't get a refund, OR you can change to Classic by using the "manage subscription" or whatever it is, but it won't take effect until your next renewal date. The person I chatted with said they are getting a lot of comments about the change and they are "taking note" of it.
There should have either been some communication from MS about this forced change coming out and how to work around it, or there should be an "opt out" option built in without having to jump through the subscription thing. The only reason I knew about in the first place was from an article on CNBC.com, it wasn't from Microsoft, they apparently know better than I what I need, and AI isn't something I need - ever.