A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Microsoft actually doesn't sell standalone Excel or PowerPoint subscriptions?
Do you see that in your account actually at https://account.live.com > Subscriptions or Order History? If so, you cancel it from there.
But if you don't see that in your account, any chance you purchased it from a third party reseller, such as Google Play on your Android device, or Apple App Store on your iOS device? If so, you need to cancel it with them, and request your refund from them since billing is processed by them.