Shared Mac in Finder sidebar

I have reset my old Mac prior to passing it on to a local charity, I reinstalled High Sierra and when I booted it up for the first time it immediately reconnected me to my network (I had to connect to wi-fi to reinstall the OS).

I have removed the email accounts, changed the username and password and disabled iCloud but there's still a shared computer with my name in the Finder sidebar. How do I remove this permanently? I can hide it but not remove or rename it. Any ideas please?