I do not agree with the tone of this topic, not only for the fact that there are hundreds of these exact same griping threads all over this site, do we really need another? But also for the negativity it will inspire as a discussion.
It is not okay to publicly talk badly about members of the forum, even if you don't mention names. Somebody you rped with comes into this thread, do you honestly think they don't know who you're talking about? How do you think that will make them feel? Not only does it inspire a negative and exclusionary atmosphere for something that is literally par for the course in rping(it happens to everybody and I guarantee you, everyone has done this at least once), but it also inspires drama and defensiveness in those who are the "unnamed" yet obvious targets of the discussion. We don't need to start arguments here, so, if you have something like this to say, I'd suggest maybe making a journal about it. As a discussion though? No. This topic has happened numerous times in the past and it will spiral out of control.
As for advice on your gripe: get over it. It is annoying and it is frustrating. You are taking a risk when you share ideas with someone in a public sphere and you don't bind them with a legal contract. If you can prove that they stole your idea, then report it, I guess. But honestly, in a place where people are just trying to have fun and multiple people can have thousands of different takes on one idea, I'd say it's a shitty thing to do. As for advertising for the same idea? You weren't the one they wanted to do the idea with. There's no nice way to put it and putting yourself on the spot for something that is probably embarrassing to acknowledge, isn't a good idea, since that is the logical conclusion we'll all come to when reading this thread. Somebody had an idea, tried it with you, decided they didn't want to do it with you and now you're whining as if it's their fault. Especially not if you want to make yourself look good to prospective partners. People do search other people's posts before partnering up and if they look and see this passive aggressive behavior, how does that make you more appealing? If I were to consider role-playing with you, I'd be worried that if I ever did something wrong or if we just didn't mesh well and parted ways, you'd make a public stink about it and paint me as the villain for making personal choices(which I am fully within my rights to make).
Just stuff to keep in mind, things to consider and people's feelings to be sensitive to in the future.