unwanted device intrusion

Have you changed your password for your account and its linked email address since this started to make sure that your account is secure.

I've done all of that multiple times

https://support.microsoft.com/en-us/help/4558767/microsoft-account-manage-your-devices#remove

I hope you find what you're looking for in this link, have you tried contacting microsoft support too?

go to https://account.microsoft.com/security/ and click on Sign-in Activity to see when and where your account has been signed in.

if it shows activity that is not you go to https://support.xbox.com/help/account-profile/manage-account/compromised-account-solution and
follow the instructions there to secure your account and prevent this happening again.

if that doesn't help click the Contact Us button on that page

select Family and Online Safety as the category

select I Want to Protect My account as the topic

skip the notification

scroll to the bottom of the options and select Chat Online or Request a Call

Hello there,

I understand your worries and it's really important to fix this problem for your security.

As said before, there are several ways to make your account more secure, I'll sum some of them up in order of execution:

1) Change password to your account

Try using a new one or something not similar to your old password. I also recommend to do a check on
Have I Been Pwned Passwords Database. This site is secure (hosted by a Microsoft Regional Director) and helps you check in all the stolen email and passwords listed in famous
Data Breaches documents on the web.Keep in mind that if your password is listed, you can be vulnerable to hackers using automated programs to login into your account. So don't use EVER any password listed in the database.

2) Enable two-step verification
(Guide)

I learned it the hard way: enable 2FA authentication to login on every
account/site with personal data or payment cards stored. Combined with a good password it will slow down every unwanted try of login by other people.

3) Remove all devices (Guide)

Just to be sure, remove access to all the devices logged in your account. It's tough because you'll need to re-do login on all of your hardware and with 2FA the process will be more slow, but safer for the future.

This is my personal and basic way-to-go for these type of problems, keep in mind that I'm not a security expert but it's a great starter. Try following it and keep an eye on new strange activities from now, if it happens again contact support.

Best wishes,

Francesca