My wife passed away. How do I shut down her online accounts?

I am sorry for your loss, if you do not have access to your wife's Microsoft account, then the information Microsoft provide for accessing a Microsoft account after someone has passed is provided on this link, they take account security very seriously and it is basically a legal process that must be entered into to access the Microsoft account.

https://support.microsoft.com/en-us/accounts-billing/manage/accessing-outlook-com-onedrive-and-other-microsoft-services-when-someone-has-died

If you wife had any subscriptions associated to the account ,it would be best to contact your bank to cancel the direct debits there.