Sending, receiving, and organizing email in Outlook on the web for business
Hello
Is this your personal outlook.com/hotmail.com email account? If so, unfortunately currently there's no way to turn this off. As independent forum user, I can only inform you of your options but have no say on the products. You can consider submitting a feedback to the Feedback portal to request this as a feature. https://feedbackportal.microsoft.com/
If this is a work/school assigned account, your IT admin user should be able to create a mail flow rule via the Exchange Admin to turn this off. Regular users don't have access to do this.