This was a criminal act - and needs to be treated as such...
I don’t see that it’s a criminal act; maybe, but you’d have to tell me the law (RICO and/or Conspiracy of some sort?) It’ a tortious act, however; Defamation, Prima Facie Tort, and maybe one of the “business” torts in the “Intentional Interference” category - Intentional Interference with Prospective Advantage, or Intentional Interference with Economic Relations.
All of this is assuming that the IP address(es) of the racist posters can be traced to Parry or his cohorts.