Report a player/client to the host and explain the reason to why your reporting them.
{"msg":"report","instance_guid":"my_session_id_here","against":0,"reason":"Breaking rule number #1"}

The server will then notify the host that a player/client has reported someone in the session.
{"MSG":"filed_report","By":1,Against":0,"Reason":"Breaking rule number #1","IP":"*.*.*.*"}