Using Outlook for iOS for personal email and calendar management
I understand why you're worried. Before closing the account, I’d recommend trying to recover it first so the hacker can’t keep using it. You can start here: https://account.live.com/acsr
If you regain access, change your password, check your recovery email and phone number, and sign out of other devices. If you still want to close the account, Microsoft has instructions here: https://support.microsoft.com/account-billing
Keep in mind that closing your Microsoft account can also affect other services linked to it, such as OneDrive, Xbox, and Skype.