In order to report a player, contact a moderator in discord with a direct message.
You have to provide evidence (screenshots or video). The in-game playerlist must be included in the evidence, so that we can see the ID of the reported player.
To display the playerlist on your screen you have to open the console by pressing your console key twice (default key is ~). Then type the command Playerlist and press Enter. The list of all players in the server and their IDs will be displayed in the console.

More specific instructions:

To report inappropriate chat behavior: Provide screenshot(s) (or video) that prove the violation. One of the screenshots must be showing the playerlist. To take that screenshot use the command Playerlist in the console, as described above.

To report cheating, teamkilling, or other forms of griefing: Provide video footage that depicts the violation. The playerlist must be included in the same unedited video (not in a separate screenshot). To include the playerlist use the command Playerlist in the console while you are still recording.

Link to Mordhau Discord:
https://discord.gg/mordhau