When you're living with your parents, it's whatever tehy want to do. You can't really argue anything unless you're paying rent. In general, they feed and house you for free, so if they want to put some fish in a tank that's "yours" (and I say "yours" in quotation marks, because if you're living with your parents rent free, EVERYTHING is essentially actually "theirs."), you can't really do anything about it i nthe end.
If you're paying rent and buying your own food, that's a different story. Just move out if you've got issues with your parents. That's what my brother did back when we were in highschool. Problem solved.
As far as people putting "unwanted additions" in my tank, I'm the only one that has any interest in my house. My wife doesn't know of any species besides what you'd see in petco, and she thinks my stock is "awesome." Plus, she knows the majority of my fish would eat anything that was put in their tank.
Although, I did see one of my drunk friends looking super guilty by one of my tanks when we had a party. My wife was like "...what did you do?" My old GT always begs for food, and he fed him some NLS. Had a good laugh about that. Drunks can't resist a fish that begs for food like a dog.