Corollary question: Will there be any way that a human visitor and/or a search engine can tell that you've deleted all articles from a particular source? If yes, you need to make it look as if you made the decision yourself for the benefit of your site, rather than that the contributor decided there's something wrong with you. ("I quit" vs. "You're fired".) But resist the temptation to imply there is something wrong with the contributor. It doesn't sound as if you really think there is-- and that kind of thing can come back and bite you.