What is spam?
Spam is unsolicited commercial advertisement. Fundamentally, spam tries to sell you something when you didn't ask for anything.
Some spam is after your money. Sometimes it's indirect, such as spam that wants you to visit a website and click on the ads. Some spam wants you to visit a website with malware that may be able to infect your computer. Sometimes spam is after things other than money; political spam is still spam.
Sometimes you'll see posts from broken spam bots that only have the framing but are missing a reference to the brand or site being advertised. Treat them as spam. Stack Exchange has some technical measures designed to limit spam, so the proportion of spams from bot tests rather than from working bots is quite high.
Some things, how ever, are not spam:
- Wildly off-topic questions are not spam. Vote to close, perhaps flag as low quality, but do not flag them as spam.
- Nonsense posts that aren't clearly the product of a bot are not spam. Flag as low quality or “not an answer”, possibly downvote (and vote to delete if you have that privilege).
I saw a spam post, what should I do?
Flag as spam.
Flagging as spam is the most effective way of dealing with spam. A post that receives 6 spam flags is automatically deleted and locked.
Do not raise a custom flag, select the “it is spam” predefined reason. That gives extra work to moderators as they need to handle the custom flag separately. Moderators see spam flags, and generally react to them by destroying the account.
Do not bother downvoting spam. It's actually counterproductive for questions as they are taken off the front page if their score drops below -4. A spam flag already reduces the post's score by 1. (But don't upvote either: we don't want to risk giving the spammer privileges, or to say to the quality control system that there is something worth saving on that account.) There's also no need to vote to close a spam question.
Do not edit the post to blank out the spam. That makes it harder for others to notice that the post is spam and must be removed pronto.
Do not try to salvage a spam question. Unlike questions that are merely badly asked, the asker won't care. Furthermore, it throws off the spam filter (see below). If a spam question inspires you a legitimate question for the site, post it separately.
I saw a spam suggested edit, what should I do?
Most spammers post questions or answers but some try to sneak in spam via edits.
Reject spam edits, or edits that are testing bots, as vandalism. There is currently no separate tracking for spam and other kinds of vandalism.
Pay attention to edits that add links — most are helpful (adding a link to a reference site, to Wikivoyage, etc.), but occasionally the link is to a spam or fishing or malware site, so check the targets of links when you review suggested edits.
How can I help fight spam most effectively?
Flag as spam and move on. Really, that's the most helpful thing.
There is one other thing: if other people with a Stack Exchange account are around (in real life, or in chat), you might ask them to flag as well, especially in the slow hours when there are few people around on the site. Anyone with 15 reputation can flag. If you have 200 reputation on one Stack Exchange site, you get an association bonus which is sufficient to raise flags on any site (if you've never interacted with that site before, you'll need to create an account on that site by clicking the “login” button).
By flagging spam as spam, you aren't just helping to get rid of a spam post. There is an automatic filter which learns when posts are deleted via spam/offensive flags, when suggested edits are rejected as vandalism, and when accounts are destroyed as spammer/troll. If the same source keeps trying to get spam onto the site, the filter will throttle it then block it outright.