Being rejected is never nice. Thus, at least, a rejection should be a chance to learn how to improve. Editors for journals and collections have a hard work to do, by before "just" rejecting articles they might remember how they felt as young students, when they first waited trepidantly for an acceptance from a journal. A few explanations make it easier to accept a rejection and most of all suggest the author how to improve.
Thus, if you are fed up of unexperienced authors or methodologically unsound articles, don't just reject them. Explain why you are doing it. How could their authors understand what they need to improve, if not at the time of a rejection?
(Personally, I usually find myself writing sentences like "your methodology is not clear enough", "you are too much focused on a single instance, generalizations out of it are undue", or "explain why your case-study is relevant".)
Have you ever rejected an article/had an article rejected in a meaningful way? What did you write/receive?
On articles' rejection, see also this post. The issue has been raised again by a question on academia.
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