Blocked user keeps coming up as online in notifications

Also keep in mind that if you send messages to a blocked account that unblocks it

That seems like a strange issue, I don't really know what could be causing it, but there's a simple workaround just in case there isn't a way to fix it. If you set your online notifications to only pop when favourites come online, it should eliminate the
issue.

https://support.xbox.com/en-US/xbox-one/console/pop-up-notifications

Click on the "Friends and Broadcasts" tab, scroll down to the bottom where it says "Show for" and switch it from "All Friends" to "Favourites only".

Also, double check your friends list, it's possible that blocking somebody might not remove them from your friends list (I would have assumed that block would also delete them, but you never know these days).

Yes I have tried blocking the users on my xbox one that's how blocking works isn't it? I did that and the notification STILL pops up when they come online and they aren't even on my friend list nor am I following them! It's so annoying.

You can turn notifications off completely and that should keep beacons from popping up. Then just pop open the guide after an online match or during a slow period to check for messages or who is online. They need to fix part of notification options as well
since it blocks achievements notifications from popping up.

SSB, that happened to me just yesterday. I keep getting a notification pop-up when Rippenharra comes online, but he isn't listed under my favorites, which are the only ones that should give me a notification pop-up when they come online...