A professor stole my dissertation topic. Now what? After completing my dissertation proposal, I got data from a company for my idea. After that, I was searching to find an advisor for my dissertation.
I talked with an assistant professor. Let's call him Alex. He asked me to send all of the collected literature and the proposal to him.  A few days later, he informed me that he could not accept me as his student.
I found another professor (Bob) and then began to work with him.  However, my advisor forced me to give up the proposal without any clear reasons.  So I needed to give it up because for a month, he kept asking me to find a different topic.
About 2 years later after the time, I found that my advisor and his former student (Carol) had been working on my proposal topic and began to submit several papers.
The story was that: His former student was in a relationship with Alex, whom I initially contacted before starting to work with Bob. Alex sent my stuffs to Carol and she contacted Bob to ask for his help to develop the idea.
Such a thing happened one more time as well.  Carol was unable to find a topic and so Bob had provided my research ideas to her again.
This Fall, Carol tenure will be under review.  I'd like to send a letter to dean of the department to inform this. However, I am unsure how to do that effectively. I appreciate any suggestions on this.
By the way, I do not work with Bob anymore. I had a chance to talk about this matter with Alex. He said that he just shared the proposal with Carol because it was an interesting idea.  He did not know the other stuffs and why and how she worked with Bob. Finally, Bob said that it happened because Carol brought the idea to him before I met him.
In two of the answers, people just suggest you to move further and forget about what happened. I disagree with it and think that if you truly believe that people took advantage of you (in my answer I assume that you described the situation in a correct way), you should stood up for yourself and be firm.

I do agree with D.W.'s answer that your chances are not high, and not all wrongs have a good remedy, but I think that you have to report it because: 


how the next potential students of A and B know that they are not people who should be trusted?
when you hear a shot and a sound of breaking glass in the night you call a police, not because you think that your information is enough to find a culprit, but because you do not want this behavior to continue. Whether they will be able to find a person who did it and whether it is really worth of opening a crime case is not your responsibility.


Sorry, but I am unable to give you a suggestion as to how approach this situation, but please do not give up.