When facing a thorny problem, I like to start by brainstorming, listing every approach I can think of. (The second step is to see if any of them can easily be eliminated.) Here's what I've come up with:
Meet with a dean, with an informal concern.
Approach the press.
File a formal (internal) university grievance, based on your university's specific policies.
File a complaint with the U.S. Department of Education's Office for Civil Rights (OCR).
If your university has a graduate student union, file a grievance through the union.
Did I leave anything out?
Now for a commented version of this list.
1. Meet with a dean, with an informal concern.
(a) If you're unsure how well protected you'd be in practice, it might be good to find out what other students' experience has been with this avenue.
(b) It might be helpful, in deciding whether to use this approach, to check how much grant money the professor is bringing into the department, compared to other professors.
(c) Even with an informal approach, it would be good to read your university's policies very carefully before meeting with anyone. The Graduate Students United at the University of Chicago has published advice for the specific context of discrimination, sexual harassment, and assault, that strikes me as good advice in general: "In making a report or complaint, or even in informal advising, describe your situation with language that mirrors the language in the University policy. Using such language in email communications is particularly important."
2. Approach the press.
If you want to pursue this, it might be helpful to find a couple of similar cases, perhaps in another department, and to identify some students who might be willing to speak to the press, anonymously if necessary. In other words, some preliminary legwork on your part would make this project easier for a journalist to tackle.
A letter to the editor could draw attention to the problem and raise awareness. It might be possible to ask the college newspaper not to publish the author's real name.
3. File a formal (internal) university grievance, based on your university's specific policies.
See comments for #1.
I took a quick look at part of the University of Chicago policy, and saw a few sentences that started to go in a helpful direction, talking about imbalances of power, but my quick look didn't turn up any slam dunk language. At any rate, for this approach, I think you'd have to work from your own university's specific policy language.
4. File a complaint with the U.S. Department of Education's Office for Civil Rights (OCR).
This approach is limited by OCR's mandates -- see http://www2.ed.gov/policy/rights/reg/ocr/index.html.
Here's an example of something that would fit OCR's mandates: if a female student were to notify the professor by email she is pregnant, and the professor were to kick her out, also by email, because of that. I'm not sure if OCR would be an option for a male student who announces he's expecting a baby. One could ask them, though.
5. If your university has a graduate student union, file a grievance through the union.
I didn't see anything in the OP's problem description related to employment as a teaching assistant or research assistant, so I feel doubtful about this approach. Nevertheless, it couldn't hurt to talk to a union representative, to at least get their perspective.
6. Guerrilla tactics.
For example, flyers posted in public places warning people from joining the professor's group. I think this is too risky.
Another possibility would be to post online, e.g. ratemyprofessor.com. This seems like a safer option than flyers on bulletin boards.
General note: for all of the above, the whistleblower's position would be strengthened by numbers, i.e. by other students getting involved.