Better late than never, it's RULE #6 (awesome reverb) #6 #6 #6!
6. Plagiarizing other people's works will result in a deletion of the materials in question and account suspension.
Plagiarization is taking the ideas of another and claiming that they are your own ideas. In it's most evident form, it's a person copying chunks of a written work in their entirety, without attribution, in an attempt to pass it off as his or her own writing. It can be more subtle, such as using a whole sentence here or there without quotes or even using a single source and simply re-wording it. This last example is why college courses require research papers to have two or three sources minimum.
It could be said that almost everything we write is a little "plagiarized" as it is all subtly influenced by things we have heard or read, but it is almost always a co-mingling of these things, not a word-for-word recitation of another person's works.
While there is no law against plagiarization, it is the stance of Atheist Nexus that it is unethical and won't be allowed.
This will depend on the severity of the plagiarism. To actively pass off an entire work by someone else as ones own is so dishonest it could easily end in a ban. The passive cut-and-paste-but-forgot-to-attribute kind of plagiarism will likely result in a warning. This mistake happens, but it shouldn't happen a lot.
It is important that you attribute your sources, people deserve credit for their work!