I don't want hate mail on my box, personally. I don't want people writing it, either. But I think you are proposing a censorship I cannot support.
People have a right to hate, as ugly as that sounds.
They must be convinced not to, by persuasion and example, not regulation. But if this space is the equivalent of a speaker's corner, then I don't want to exclude racists. And, the rec. Let's make this a crushing defeat for racists.