I do think those users should be warned and/or punished.
Remember that spamming is posting without substance and if they're not willing to at least add to a discussion then the thread is pointless. It's like if you were to post a lot of threads in Economics about the bitcoin price, every hour. Useless.
Besides that, I think the forum should require a specific format when citing an external source(or the forum rules should require users to specify the text he/she copy pasted in the cited link by highlighting the copy pasted text in bold) and should require one to have further discussion besides the copy pasted content in an article in order to prevent others to spam copy pasted post from cited articles.