I encountered a similar situation modding /r/moderatepolitics. A user was flooding the subreddit with posts from right wing websites. They had the explicitly stated aim of tilting the predominant atmosphere on the sub towards being right wing. We eventually found the perfect rule: require everyone who made a post to provide a substantive starter comment, one that didn’t just summarize the article but instead was enough to start a discussion. This not only helped get conversations going, but also raised the bar to posting just enough to discourage article dumps.

That said, I think overall low traffic in /c/europe is the core problem. No rules short of outright censorship can help that.