This is because the Builtin\Administrators group is a member of the sysadmin server group, which gives any administrator account on that PC unrestricted access within SQL Server.
You can remove the BUILTIN\Administrators group from the sysadmin group by:
1. Opening Enterprise Manager
2. Open your server, then Security and Server Roles
3. Open System Administrators, then remove the BUILTIN\Administrators group from the list.
I'd be very careful about this though - if you don't know what the password for the sa account it, you will lock yourself out of the system administration functionality.
John