assuming you have proof other than your own word that postings his (he may have used university account, in which case information about his use of account may well exist), send word of business about his student to chair of his department, his dean, and university president. especially if he not tenured, he might have big problem. (and were you his student at time of hanky-panky?) just be sure you have backbone to stay course, and to tell only absolute truth.
or, you could tell him that what you do if he not take down everything he said about you. keep in mind that letter from lawyer can do absolute wonders.