[bebe3000]

How about creating rules?

  A talks to B
  B blocks A
  A registers as C
  *not too much time passes*
  C talks to B

At this point C is considered to be a harasser and therefore it will be hidden for B. B can of course lift the ban if it's just a false positive. To further specify who's who in this graph, the logic can find the smallest time between ban and the creation of a new user with the same behavior (writes to B) as the reported one.

I think this is way more easier than handling those who abuse the actual service you're selling.