The thing is, it takes more than "copyrighted content" in order to be actionable. I mean, you can sue anybody for anything in US courts, but it's not apparent from your question that they are taking entire articles, which makes a big difference to the kinds of options you have to deal with it.

If it's entire articles of content for which you hold copyright, you can certainly send a DMCA Takedown notice to their ISP and start the ball rolling that way. Warning: may require lawyer. If they aren't taking whole pieces, then your course of action can get murkier and will definitely become more expensive.