Hi Leonie!
Unfortunately, there's no way to do this out of the box in Jira.
There's a feature request - JSDCLOUD-4721: Restrict Remove Organization which I encourage you to watch and vote for.
Unfortunately, I can't provide a timeline as to when or if it will be implemented, so your best course of action will be to watch that issue for further updates.
As a workaround, my first thought was that we could remove the permissions for modifying the participants of an issue (since that's where they come from) but there's no permission to control that to the granular level you need. Customers can either modify that list, or they can't.
I also took a look over at the Marketplace to see if there were apps there that might help you get the functionality you're looking for, but I haven't found anything that might be of use to you.
Hope this helps!
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