Untrue negative comments on RateMyProfessor website
As the comments suggest, just ignore it. You will always have disgruntled students even if you do a great job. Some will blame you for their own shortcomings. 

But the best "defense" against such negative comments is the positive comments from those who think you served them well. If those appear also, then a reader can easily judge it for what it is. 

If you want to "sabotage" such things, just point other students to the site, without comment. You don't need to "defend" yourself when your other students naturally do so themselves.