Amateur Creative Ip Advice

Hello there!

I’m not entirely sure if this is the best sub to put this in(not much of a redditor myself so if there’s a better place to point me towards, feel free to let me know) but I’d appreciate some advice either way!

Here’s how it is: A bunch of my friends and I are writers, largely of amateur content such as TTRPG campaigns, written stories, and worldbuilding projects in various stages of completion. A few months ago, one of our friends, we’ll call him Chris(20s M), and his now-ex Morgan(20s F), broke up, and this precipitated Chris falling out with the rest of the friend group over differences I can expand upon if needed, but I don’t believe to be directly relevant to this post.

Here’s where things get tricky. Chris is the most active writer in the friend group, and has written several novel drafts featuring many characters, ideas and plot points created by other members of the friend group. He self-published his first novel prominently featuring one of Morgan’s characters. Upon realizing this, Morgan requested that he change the character if possible(he refused, as the novel has already been published) and that he refrain from using our characters and ideas in his future projects. Even though we have proof that Morgan is the creator of this particular character, I’m fairly positive there’s nothing to be done as far as that particular novel goes, but for our other projects that he has been part of there is quite a bit of documentation on who contributed what and to what degree.

My question is this: What, if anything, can(or should) we do to safeguard our work from being used in any future projects of his?

To be clear - None of us have any problem with him using the aspects of our collaborative work that he himself created. That is his right, as far as we are concerned, and we aren’t trying to restrict his ability to pull ideas from these projects so long as those ideas were his in the first place. My question is only concerned with whether or not we are able to insulate ourselves and our work from the possibility where Chris may use our ideas without our permission after being explicitly asked not to. Any or all advice would be greatly appreciated! Thank you!

Location: MA for me, TX for the rest of the friend group