Another great comment of @alexgr I'd like to contribute the following to:

• Spam is "permanent" damage to the system, since it is backed up by a blockchain
• Plagiarism can get you in a lawsuit & can cause the company Steemit problems (DMCA), to further explain. Your taking someone's work without their permission and making a profit on it, that is punishable by law.

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.}