Every post, reply, comment and profile has a three dots menu. It offers Report, Hide for single posts and comments, and Block for accounts. Hide and Block only change what you see. Report goes to the team, who review every report.
To report, describe what is wrong in 20 to 1,000 characters and press Send report. Follow your reports in Command Center, then Support and safety, then My Abuse Reports. To unblock someone, open Settings and use the Blocked accounts card.
For impersonation or stolen identity, use File Abuse Report on the Support and safety page.