At the bottom of posts there is a "Report Post" button. It's a rectangle with those words and a circular icon with an exclamation point symbol. On mobile devices you may have to click the Tools option (with the wrench icon) and then select Report from the menu that pops up. You can also report a member's behavior with the Report a Member link found on the Home page, or report directly from their profile.
For further information on how to report members, read this Resource.