You can report a player using the in-game "Report User Abuse" Button. This button is located below the chat tabs when they are visible. (see below)
When pressed, the "Report User" window will be displayed. Select the name of the character you wish to report and choose the correct reason. You may also type in a brief description of what is being reported.
You may also report players through the Mabinogi forums. This can be done from the "Contact Moderator" section which can be found here.
All information posted is not visible to any other players, and can only be viewed by the various forum moderators, and Game Masters.
When reporting through this forum, any video or screenshot evidence must be provided for an investigation to be conducted.