Spam is definitely discouraged and also has the potential to harm one's reputation.
Plagiarism, if done maliciously (pretending that the content of others is one's own), can also lead to harm in one's reputation and is generally perceived negatively.
If the content that one is posting about is not their own they should at least provide a credit and/or link to the original source.
Another great comment of @alexgr I'd like to contribute the following to:
Tip:
If you still wanna post something you haven't made a proper way how to "avoid" plagiarism is to link to the original source and add a "personal twist" to it.
Personal twist - Stating your opinion about the subject eq. adding your "stamp" on that, adding more info etc...increasing it's value.