Nope, sorry, destruction of other peoples' property is not protected under the First Amendment. If you take that argument to its logical end, nobody would have any property rights at all. If you take it to the extreme end and consider the body itself to be property of the individual, than battery and assault would have to be decriminalized.
Let's do an experiment: The next time you want to get home in a hurry, mash the gas pedal. When the cop busts you for going 110 in a 25 zone, just say it's your freedom of expression to drive as fast as you want. Somehow I am betting you still get the ticket.
This reminds me of people who took flags off of flagpoles a few years ago and burned them, then argued that it was their first amendment right to burn the American flag.
I might be persuaded to listen to their argument if they were talking about burning THEIR OWN flag, and not stealing somebody else's (or the public's) flag and burning it.
This professor had every right to set up her own display, even make it identical to the pro-life display, and then destroy that. Or she could make a counter display. But she has no right to destroy somebody else's display.
Ayn Rand said 35 years ago that this was basically the goal of the Left, and one of the means by which they'd attempt to implement it. I remember her essay on the Free Speech Movement in 1969 or so.