I think a critical point is that while harrassment and hate speech can be virtually identical in content, the difference in proximity and targeting is in fact the critical one. A hateful billboard or a hateful speaker at “some rally downtown” is much easier to ignore than 100 hateful letters in your mailbox, or 100 hateful protestors outside your house. For one individual harassing another, the legal remedy is clear. Slightly murkier, but still possesing strong legal remedy is the case when an coordinated orgnaized group (a corporation, club, gang) is harassing the individual. The issue is thornier though when a number of unrelated and unconnected individuals act independantly in such a fashion that the Cumulative effect becomes equivalent to harrassment.
If I were to blindly dial a number from the phonebook and say “You are a terrible person. I hate you!” no crime will have been committed and the recipient may be nothing more than confused. But if 500 people all get it into their minds to call and speak the same thought, the same person might reasonably be justified in leaving town in fear of their life. The statement “I hate you!” will never be bannable while the first amendment stands, yet nonetheless it and its variations comprise the bulk of online hate speech and harrasment campaigns.