I agree that freedom of speech should be held up as high as possible, but I also think that hate speeches should be forbidden. In order to achieve that, it is paramount to know where the exact line to a hate speech is. Someone saying "gas all jews" is past this line (although as a German I may be overly sensitive on this specific topic). For me the difference lies in voicing your opinion ("I don't like this group") vs. incitement of the people ("Kill these people, because I don't like them"). However, I also have to agree with rich_is_bored here: This is something people should do out of their own volition. In my opinion the approach most likely to succeed in this regard is to educate people; to show them where there are differences and why these differences do not matter. Ignoring differences is just as bad as pointing them out as they DO exist. The thing is you have to accept them and know how to handle them, not ignore them. As an example, the latter would be to serve Jews and Muslims pork at your dinner with a side of beef for your Hindu friends. This will cause trouble, because you ignored their religion. On the other hand, you can use the differences to your advantage. For example: if you have a company that has to be available 24/7 and you have a good mix between religions, all Christians can take a vacation on Christmas as other religions won't care for it. At the same time these Christians can work on holidays of other religions.
The idea that words cause violence, and need to be censored in order to prevent violence, has never been supported by any convincing evidence. It is mostly based on correlation fallacies: "Bob abused his ex-wife online, then he assaulted her, therefore words cause violence and censoring his online speech would have protected his victim". Most speech, even offensive speech, is not a prelude to violence, and even if it were, this wouldn't prove that the speech causes violence. This is basically the same nonsense argument that we saw in the last century, when religious whack-jobs were trying to ban comic books because they might make children violent.
I get your point that in most cases verbal abuse does not cause violence, but they are rather a symptom of the same problem. Still, there are definitely cases where hate speeches have led to violence; the Third Reich is a perfect example: without Hitler's invitation to act against the jews, it would not have been done (e.g. look up "Night of Broken Glass"/"Kristallnacht"). Usually, this includes the explicit request to act against someone (hence the differentiation I made earlier between opinion and hate speech). If you use stuff that could be considered as hate speech as a joke, you should also make clear to everyone that it actually is meant as a joke (and even then there will be idiots, who will think you are serious).