Block access to Western Digital MyCloud from the Internet

Western Digital’s NAS - called MyCloud - automatically makes itself accessible to their cloud-based application. So the user can log in to WD’s website and remotely see all the content on her or his NAS, but someone else might be able to hijack the connection. I do not want a remote site to be able to access my local storage. Not even a site that requires a password of me. I can’t find any way in Avast to block incoming access from a specific web site. What am I missing?
Thanks.