The point that has not been substantively addressed, at all, is that Greer's talk was on women's place in politics in the 20th Century. There is nothing in the title of the talk that promises comprehensiveness, making "the exclusion of trans women" a not particularly compelling reason why she should not be allowed to give the talk, and while at least one person says that she'll almost certainly say anti-trans things during the talk, despite it not being germane to the topic - a claim that is pure speculation, by the way - it has not been explained how, exactly, this would render moot other potentially enlightening commentary on the subject she might offer, nor why the protest has to be centered around outright excluding her, rather than simply, y'know, creating awareness of her views and then letting other adults make the decision of whether or not the speaker having uttered such views and/or the possibility that they might subtly or overtly creep their way into the talk is enough to persuade them not to listen to her.
I'm all for protesting her if you think her views merit protesting, but there's a difference between fighting ideas with ideas and fighting ideas using the mechanism of power to try and stamp them out from the top-down. That may not be a technical violation of freedom of speech (though if the university is public, it might be), but it's certainly a violation of the principle inherent in free speech, which is that ideas deserve to be stamped out by other ideas, not by fiat. And the idea that one set of bad opinions necessarily renders worthless all other opinions a person might tender is pretty much diametrically opposed to the classical and valuable role of higher education as the crucible ideas, good, bad, odd, and indifferent, of the past and present.