But they have that right. It's political speech with a small "P," however uncharitable. And note that it's being punished for political, rather charitable reasons.
Ask not for which unprotected class, such as "frat boys," the right to free speech is withdrawn. It is withdrawn for thee.
The frat boys have not been denied their right of free speech or any constitutional right.
They are of course free to sue OU, but they would likely lose after taking the stand and making asses of themselves.