Every once in a while we get a question about how to report a bad user to the admins. Sometimes it's a spammer, sometimes it's a troll, sometimes it's just someone who had a momentary blip in the "socially acceptable" filter. But the answers are pretty much always the same. So, here is a "master post" for this sort of situation that we can refer to in the future.
[some user] is a spammer. What can we do about this?
[some user] is being blatantly offensive. How can I report it?
[some user] is personally attacking and/or serially downvoting me. Can the admins do something?
Almost all of [some user]'s answers link to the same product, but some of the answers are upvoted and/or fairly helpful. Is this spam? What should I do?
I'm on the fence about making this an FAQ proposal. If someone else thinks it's a good idea, it's already CW, feel free.
Flag the behavior. You can mention in the flag comment if it's a pattern of behavior by the same user. Please, do not call out the user by name here on Meta. If it's persistent bad behavior by a single user, then you can use the contact us link at the bottom of every page.
There are lots of ways to handle these situations. The admins tend to take the view that behaviors, not users, are bad; that said, they will suspend users in extreme cases.
Flag for a moderator
You can send short messages directly to moderators by clicking the "flag" link under posts or the little flag icon next to comments and choosing "Flag for moderator attention."
Flag, but not for a moderator
If it's truly blatant spam or hate speech, you should be able to just flag as such directly. Enough such flags and the system will take care of the problem automatically, no need for any time-consuming human effort! Beyond the 1.5 seconds it takes to do the actual flagging, anyways.
Send e-mail to the admin team
You can always e-mail via the contact us form with your concerns. That link is in the footer bar of every single Stack Exchange web page.
Post to a meta site, like this one
This is generally discouraged. Since everyone can see what's on meta, calling attention to problems this way has the feel of public shaming even when that isn't the intent. Not exactly in the spirit of privacy or "focus on the behavior, not the user." Still, there are cases where this can be useful, such as spam (which isn't posted by "real" users and can be cleaned up without mod help), or odd activity that isn't linked to specific accounts.
What you shouldn't do is e-mail a moderator directly. Despite many of them listing their contact details in their profiles we need to have all site related communications on site where they can be reviewed by other moderators or the Community Managers.