8 of 75 see http://meta.stackoverflow.com/questions/56223/has-there-been-an-uptick-in-spam-votes-against-established-questions/56224#56224

What is the effect of the Spam flag?

The spam flag is designed to eliminate posts with no relevant content and to penalize the authors:

  • 3 flags (spam or offensive): post is banished from the front page.
  • 6 flags (spam or offensive): post is locked, deleted, and the first revision owner loses 100 reputation.
  • Because a question with 6 flags is locked, a 10k reputation user cannot meaningfully undelete it.
  • Each spam flag counts as a downvote for calculating the post's score. [Copied from offensive flag info--is this true?]

Any user with at least 10,000 reputation can see how many spam flags a post has accrued via the "tools" page.

When should the Spam flag be used?

A question should be marked as spam ONLY when it consists of undiscriminated bulk advertisement. It should NOT be marked as spam in the following cases:

  • The answer contains no useful information (such as an answer that says "I don't care about your problem")
  • It contains gibberish (such as "fsdguejgkfdlk")

(Source)

Is there any way to remove Spam flags?

There is often no need, as spam flags expire after 48 hours if the thresholds aren't reached.

Rolling back a post to a previous state will revert to the number of spam flags from that particular revision. This allows the OP (or someone else with edit rights) to rollback a post that someone else made spam in a later revision. However as a general user, once you mark a post as spam, you cannot take it back.

How does the Spam flag differ from the Offensive flag?

There is no functional difference aside from separate counts - 3/6 of either will be sufficient for mechanical purposes. 10k users and mods can see which flags are applied, and Jeff's plan according to a comment on a blog post was that in the future posts that are highlighted as spam might be used to help construct better measures to protect against spam.