I haven't heard of anyone stealing my stuff on this site. Not unless they prevented me from learning about it.
I have had people, in my life, not on this site, take ideas from me in ways that really bugged me. For a time when I was younger I wanted my pen name to be "L.D. Kimberly" which is my first and last initial, then my middle name. And I had a friend start calling herself "T.U. Marie" and everyone telling her what a clever idea that was... that miffed me quite a bit. And I have had online writer friends blatantly steal my titles, or say I wrote a story based on a song lyric, they would write a suspiciously similar story based on the same song lyric... things like that are just bound to happen, and for some reason, the person who steals you crap always gets more recognition than you ever will.
But that's my tirade. No, nobody HERE has ever ripped me off that I'm aware of.