curl -sS -X PUT 'https://matrix.example.com/_matrix/client/v3/rooms/!abc123:example.com/redact/$143273582443PhrSn:example.org/txn1' -H "Authorization: Bearer $ACCESS_TOKEN" -d '{"reason":"spam"}'

Replaces the message content with a notice that it was removed, keeping the event's place in history. The reason is attached to the redaction. Requires sufficient power level, and the transaction ID prevents duplicate redactions on retry. For bulk cleanup use the admin redact endpoint, which can redact many of a user's events at once.