If you've worked in education long enough you've probably run across a student who for one reason or another decides they don't like you, or something you've done, and they want to let you and maybe lots of other people know about it.
I had my own brush with this experience in 2010, when a student decided they had an axe to grind with me and proceeded to post a very biased and negative article about me online. What I can share from that experience is that the best response is usually no response at all. It is frustrating that the internet is a wild west of anything goes-style free speech, but when all is said and done, most of these situations are only as serious as you make them - in other words, treat it like a tempest in a teapot and you will generally find that indeed that is what it is.
Another suggestion for anyone, regardless of whether or not you've been subjected to an online attack, is to control your own information. In other words, create the content that you'd like others to see instead of leaving it to chance and what is already out there. This may sound like a lot of work but it can actually be a great writing and thinking exercise.
Hope this helps!