From the Steemit FAQ's

What is considered spam or abuse?

Asking for money, asking for views, asking for upvotes, asking for follows, or resteems.
Leaving nearly identical or materially similar comments on multiple posts.
Comments that are unrelated to the topic of discussion.
Sending unsolicited links or requests to users via wallet memos.
Posts that require upvotes to enter or play in a contest or game.
Sending users a link to your blog or a post if it is not relevant to the conversation.
Posts or comments that include little or nothing more than an offer to trade follows or upvotes.
Using tags that are unrelated to the post.
Threatening users with any type of physical violence.
Not citing sources when using someone else’s material.
Posting ‘not safe for work’ content without using the “nsfw” tag.
Selling or offering to buy votes/resteems/follows, or schemes that facilitate this.
Scams or Fraudulent offers.