system
Given the following post, respond as if you're commenting on Reddit:
human
Grad School Professor Telling People Not to Hire Student

Hello, I am in a strange position and would like to have some help/advice/insight.

I recently graduated from a Master's program.  While there, I did well in my classes (3.9 GPA) and was well regarded by students, faculty, and supervisors.  Except one staff member (the department chair) and for the reason I will explain.

There was an issue where I felt like the dean of the department needed to be brought in on a conversation.  EDIT (added for clarity): *I sent an email to the department chair and cc:ed the dean as I felt like it was important for the dean to know*.  A faculty member had made a mistake that reflected poorly on the school and I genuinely wanted to help correct the mistake.  That faculty has since been let go (for various balls dropped).

Another faculty, the department chair, felt like I had gone over his head.  He threatened to not let me proceed in the degree program because he believe it showed I wasn't mature enough.  He also said that he is well connected in the field and that I should be aware of that in the future.  He told me that if I didn't acknowledge my mistake, then I would not be able to continue in the program.

At the time, I took notes but didn't want to go any further with any complaint for fear or reprisal.  I kept my head down and finished the program in very good standing.

Unfortunately now I am applying for jobs, and I have been made aware that a job offer was not made to me because of intervention by this professor.  I interviewed, the employer liked me, went to the stage of getting reference checks (the ones I provided were all positive), and... yes, the professor was connected to this employer and he was able to stop the offer from being made to me.

Do I just chalk this up to how life works?  Is the professor within his rights to prevent me from employment based on what he has stated to me?  Thoughts?

*Edit: A few more details that have come up in comments:*

1. *I am not the only student that has been threatened by this professor.  Another student was threatened in a very similar situation.*
2. *While this professor is connected, (being vague) my fiend isn't one where one person really can ruin a career.  It isn't a typical business type career (sorry to be vague).*  
3. *I appreciate the feedback (keep it coming).  I have learned a lot from you all.  I think a few take aways for me:*
   1. *Academia should be treated like a career.  I have always gone to small, liberal arts schools where students are encouraged to speak out and dialogue is appreciated.  I guess I became too "comfortable".*  
   2. *I still believe, even if I did act "unprofessionally", the professor acted inappropriately by threatening me.* 

*I'm still processing this so feel free to keep comments coming.  I really am someone that wants to learn but I also believe in fairness.*
gpt
I'd complain above the professor's head. Academia is hard enough without someone actively trying to screw you over. Start with his head of department and escalate through to the dean upto more senior university officials demanding action. As an alumnus and graduate student of the university it's incredibly unprofessional for a professor to be actively undermining you.

Gather what proof you have about the professor screwing with you and reach out. What they're doing is certainly unethical and likely in violation of University codes of conduct.