IANAL, but there are two cases:
1. Posts with proper attribution
In the scientific tradition, everything that is quoted should be cited. If you copy a paragraph from Wikipedia, mention where it comes from. If you copy a paragraph from a book, do the same.
The owners of the content you copy have a right to have the source acknowledged. This is not only the case for obviously copyrighted material (such as from books), but also with Wikipedia articles, where you agree to share it under the same license:
Permission is granted to copy, distribute and/or modify Wikipedia's text under the terms of the Creative Commons Attribution-ShareAlike 3.0 Unported License
To re-distribute text on Wikipedia in any form, provide credit to the authors either by including a) a hyperlink (where possible) or URL to the page or pages you are re-using, b) a hyperlink (where possible) or URL to an alternative, stable online copy which is freely accessible, which conforms with the license, and which provides credit to the authors in a manner equivalent to the credit given on this website, or c) a list of all authors.
Sometimes it's inevitable to just quote a sentence or more, especially if you're citing a very important source (that doesn't necessarily have to be Wikipedia). It would be wrong to forbid that, and after all, there's the concept of Fair Use.
However, at all times there should be:
- A proper attribution or acknowledgement as demanded by the source
- Making it an actual quote (i.e. use the blockquote style, or put the quote within "…")
2. Posts without attribution
If there is no mention of a source at all, then I would try to find it, add the proper attribution, make it a quote and leave the post. This often happens with first-time answerers who never come back to the site at all. Maybe what they posted was helpful, so there's no point in deleting it altogether.
If you however see users repeatedly posting answers that include non-original content without properly citing it, I would inform them that this is against the policy. In fact, content you write on the SE network should not infringe copyright in any way. See the "legal" page:
Using copyrighted material does not constitute infringement in all cases. In general, however, users should be careful when using copyrighted content without the permission of those who created it. It is our policy to respond to notices of alleged infringement that comply with the Digital Millennium Copyright Act ("DMCA").
High-rep users should know better. If you see a post that consists almost entirely of unoriginal content, downvote, leave a comment, flag it for moderator attention. We want to create our own content, not rip it off other sites. Users should be able to re-phrase or summarize, adapt their answers to the question, and not just 1:1 copy everything.