What happens moving mail to junk?

It doesn't block it but future email from that address should go to junk.

Through the apple mail app, probably not.

What exactly that does depends on who your mail provider is. For some moving it to junk flags it as spam, for others it may not. And for it to count you'd need to check into your mail provider and probably either go in through safari (as in sign into your mail via the web interface....for example I use yahoo so i'd need to sign into yahoo.com), or if the mail provider has a stand alone app you can use