First Jura, my congratulations for your always excellent questions and answer here in italki. You are very active and you have been a great italki user. You have helped a lot of people, I think. I usually read your questions and answer and I am thankful for what you have helped me, as an English learner and new user of this website.
Now, to your question above.
Initially, I will proof that the the idea was mine. Second, I would ask for explanations from the one that have stole my idea. If the explanation is not good enough, I will tell as much people as possible about what happened. Finally, i will take all the measures in order to make justice and even to go to a Court of Justice, if necessary.
This kind of behavior you have mentioned must to be reproached. We can not just forget and let people like that being dishonest, without doing anything. If we don`t act we are supporting them. That is not fair!