Hello, interesting question.
I spent a lot of time playing 40K tabletop & games such as DoW Dark Crusade and Soulstorm in my early to late teens, so understand that it’s all fictitious and have that line between reality and not.
Moderating it though is quite different, you do get quite a lot of people who just have no understanding that it’s satire. With the anonymity the internet gives means people feel more free to voice some god awful things, but that’s true everywhere online.
To be honest I moderate it in the same way as I moderate other spaces. Create a set of clear rules for the community to abide by, and if they break those rules they get warned or are removed. Generally As long as the members are talking about the fictional events, and not relating them to everyday life, it’s usually ok.