We read your post carefully. We understand what was written.
As I wrote, there is little you can actually do about online harassment appart from reporting, and blocking. You behaviour with others will always hold stead against flagrant bouts of mud slinging, unless you follow suit and do the same in retalliation - for all you do then is become no better than they are.
Most OOC harassment starts from misunderstandings where IC and OOC are not clearly defined at the start. The other sort of harassment is generated by people who are so deep into their prejudices and preconceived ideals in the world that there is little one can do about there behaviour and attitude in game - apart from report them to the GM's and hope they are banned.
The statement 'I am afraid that good characters and innocent people are often the victim of harassment as well.' is odd... as that goes without saying - however some are not so innocent and perpetuate the situation by reacting in a similar way. The best way is not to react so, but to distance oneself from the instigator - and report them for such behaviour.
With regards to /report not working on all forms of harassment - that lies with how you report, and how you prove your case.
/ignore is only a means to distance yourself from them. Sadly we live in a society that allows free speach, and as I said - people naturally take sides. The only way do counter that is not to engage in slander. And if you hear other slandering someone, challenge them to stop it, as they would not like it done about them - if they reply - 'but he started it' or 'he does it about me' that only shows their true colours and lack of maturity.
and finally, you say "I'm sorry, but perhaps you didn't read my post closely enough. We're talking about in game, OOC harassment from people with OOC hatreds."
I was too... but I, rather than concentrate on their behaviour that is appalling, I chose to write about how to help prevent tarnishing of yours by your own actions.
I go back to reporting - if you don't report them for ooc behaviour that is 'racist' or 'otherwise abusive' and can be proved as ooc they will just get away with it... for if you get to hear about the lies, someone somewhere has perpetuated the situation by repeating it, exaggerating it, twisting it more - they are just as bad as those who started it, if not worse for allowing it go on.
I go back to my final statement - if you personally keep a clear line between ooc and ic - no-one can accuse you justly that you are using ooc hatreds in charcter, also usually those who are that way inclined will react in such a way that when being monitored after being /reported will land them being banned. [and for such behaviour I think it should be a total ban - but that is another discussion, as is my feeling that guilds should have part responsibility for 'policing' their members behaviour in game - and have rulings to punish such behaviour - but again that is another debate]