Welcome to the Microsoft Q&A forum.
I understand why you want to sign out immediately, especially since you believe the email account on your phone may have been compromised. That is a reasonable first step in this situation.
I can see there is already a Q&A Assist answer on this thread. The steps it provided to remove/sign out the account from the Outlook mobile app are generally correct. Have you had a chance to try that yet?
Before suggesting the next step, there is one key detail to confirm because the supported path is different depending on your access status: Could you please confirm whether you can still sign in to the account (for example on Outlook.com in a browser, or on any other device), or are you currently locked out?
1, If you still have access (you can sign in)
After removing the account from the Outlook app, the recommended next step is to use Sign out everywhere from your Microsoft account security page. Microsoft states this signs you out of browsers, apps, and anywhere else the account is used to sign in within 24 hours (except Xbox). How to sign out of your Microsoft account everywhere - Microsoft Support
2, If you do not have access (you cannot sign in)
In that case, the supported next step is to use Microsoft’s hacked/compromised account recovery flow, which starts with the Sign-in Helper tool and guides you based on the account’s status. Microsoft notes the tool asks for the email/phone number and then provides the appropriate self-help steps or available options. How to recover a hacked or compromised Microsoft account - Microsoft Support
If you are directed to the recovery form, Microsoft also provides guidance on completing the Microsoft account recovery form and what information is needed. Help with the Microsoft account recovery form
Please note that this is a public user‑to‑user forum, and contributors here do not have access to accounts or the ability to sign users out remotely. All account‑level actions need to be completed through the official Microsoft account pages.
I hope this clarifies why confirming your current access status is important and what the next supported step will be.