SQL Server service is running under an account that is a member of the built-in Administrators group
The built-in Administrators group is a machine-local group.  You need to connect to the machine where SQL Server is running via the Computer Management console, and remove the SQL Server service account from the "Administrators" group, as in:



Be aware, the SQL Server may, for some reason, require access to the local machine via membership in the local Administrators group.  Only you can determine if this is required for your specific instance, since it is not required for an out-of-the-box install.