Complain

The /complain command is used to alert the Ocean Masters to a player who is being vulgar, abusive, or otherwise violating the terms of service. When a complaint is submitted, the OceanMasters then review it and may take disciplinary action ranging from a warning to a ban. For privacy reasons, the outcome of the complaint is only discussed with the player who was reported. Players who submit false or frivolous complaints are themselves subject to disciplinary action.

A complaint may also be submitted using the "Help" button on the Ye panel.

An alternate action to consider taking instead of a /complain would be a /blackspot, if the player is creating a public nuisance.

Finally in the words of Bia "It really is less helpful if you petition or complain on behalf of your friend rather than the friend contacting us his or herself."

Usage /complain 

Complainspam
Complainspam refers to the use of one or more appeals to the OMs in order to secure a gameplay or political advantage that would not be available to the player without OM intervention. It includes protesting an action taken by another in hopes that the other player will be disciplined or stopped, or appealing to the OMs to undertake a specific course of action. Often complainspam implies an element of duplicity, where relevant information is withheld from the OM in question in hopes that the OM will act as the complainer wishes based on partial but misleading information, or an imputation of malicious motives that is unwarranted. Complainspam equally refers to appeals made via routes other than the /complain function, such as petition and /tell.

It should be noted that "However, if you are repeatedly submitting complaints (within minutes or seconds of each other) it will likely generate a response, which may include a suspension from the game as a time-out." and "Attempting to garner some political edge via multiple complaints or petitions is not likely to work."