Currently, the difference between official server game modes (PVE, PVE-C, and PVP) is displayed as a text while the mouse hovers above the game mode, like this:
Plenty of players seem to miss this description completely, which is bad enough for the user experience when they get the unpleasant surprise of being killed by another player. However, an even bigger problem is that some other players feign ignorance of the difference between PVE-C and PVP, and then use that to try to justify griefing.
My suggestion is to detect that the player clicked on PVP or PVE-C for the first time since installation and, if that is the case, show an additional pop-up dialog spelling out in very “loud” font what the players can and can’t do, with a link to the official Terms of Conduct that the player can click on.
This pop-up dialog could be a one-time thing or have a “do not show this again” checkbox.