You'll likely have to do it in the email settings on the server, such as if you use Gmail mark it as spam there, or if you use me.com then in the settings on iCloud.com.
Nope. I know this is how it should work but I get spam emails daily on my iCloud from the same 6 senders such as Bed Bug Defense, ADT, some hair treatment and other crap. Each time I send it to spam and the same day or even next day I get a new delivery. I think marking anything as spam in iOS is useless.
Marking something as spam is different than adding the sender to a blacklist, blocking future emails from that address. Spam filtering is not an overnight fix, it's algorithms and filters that're refined over time.