I searched for this and found the post from 2015 so I'm posting here as it's relatively recent..
There actually is a way with Gmail as I left this app that I love for that one because it had "block sender' , 'block entire domain' and 'move to spam'.
When moved to spam you actually have to go to Gmail to remove it from the 'your' spam list..
So to the point, there is absolutely nothing stopping adding a block sender/domain, if I SAY ITS SPAM they WHOEVER it is I do not want any email from whoever it is, I block Walmart as I shop online, and they love spamming me..
And as we all no ' unsubscribe ' is a 1. Pain 2. Not aslways the best idea with REAL spam...
So as I said I love this app, but I can't stand the spam.... Why I left... So I'm back so could you PLEASE, add this as an option...
Thanks,
Mike