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Is there some setting / block / third party app where I can prevent someone adding another person to group messages?
In short, no, it cannot be done.
No-one "owns" an iMessage group, it's entirely ad-hoc - so who would be the moderator of who can & cannot be in it?
What you can do is leave the room when you are invited or turn on do not disturb for a noisy room if you keep being added by “helpful” “friends”.