That's a problem I have to face as well. Even with attribution, entire (or vast parts of) articles can not be posted. Only a summary or beginning paragraph can be pasted with a link to the rest. It's a good idea to disclaim liability of members' postings in your Terms of Use, but state that you will remove copyrighted content if notified by the copyright holder.
I ask my members (and have to continue to remind them) to post one or two paragraphs... I normally allow the opening paragraph (as that normally outlines what is being discussed), and the second paragraph normally lets readers know what stance or direction the author is taking/writing the article in. And so I allow the first two and then a Source link... However I do have a number of websites who have given my website to copy full articles from their website and I have that permission in writing and saved it, and in return have given the website the same curtousey.
hmmm reminds me i have some of my full articles/forum reviews reposted on other forums with a link back. I don't mind as they usually ask for permission first and well the whole point of most of my articles is to help others with info they need.