Appearance
Report & block
Report
On a profile, post, or chat, use Report. Pick spam, abuse, or other, and add a short detail. You cannot report yourself. Repeat reports on the same person within 24 hours are folded into the open report.
Reports go to moderators. You will not get a public debate; you may get no reply. That can still mean the account was actioned.
Block
Block hides that person from you and you from them as far as the product allows. Unblock from Settings → Privacy & safety (blocked list).
Blocking is not a police report. If you are in immediate danger, contact local emergency services first.