Are you seeing SPAM around here? Here are a few notes on what to do:

If the SPAM is a post (or more) in any part of the forum: At the bottom right of every post in the forum there is a little exclamation mark button [!], which you can use to report offending posts to moderators. It will take you to a little page where you can mark it as spam and add a little description if you need.
If there are any moderators online at the moment, chances are that they have already noticed, but it won't harm to mark it for review anyway.

If the SPAM is via Private Messages:
Forward the offending message(s) to a moderator or administrator. It will be taken care of as soon as they come online.

If the SPAM is part of any user's profile or signature:
Send a PM to an administrator and mention the username with the offending content so it can be taken care of.

In case of doubt, you can always ask a moderator.