Reports of Griefers are only accepted through the in-game reporting feature.
[In-Game Reporting Method]
1) While in the game, interact near the player you want to report (PC shortcut key 'T') to expose the report button.
2) Click on the account name of the player you want to report in the chat window to see the report button.
3) Click on the account name in the User List in the Map (M) > Social menu to see the report button.
Click the report button > select the type of report > describe the report details, and then submit it.