I think sometimes, these people (whether trolling, or real) need to be scared to death. Well, not to death, but really frightened. No talking to them nicely. Find them, turn up, arrest them, slap them in a cell for as long as the law allows without a charge, and then have the biggest, roughest looking copper the force has yell at them for a hour or two.
Then, if something can be found to charge them with, charge them.
Make them think they really are in danger, and humiliate them a bit, and they might, just might, get how their behaviour affects other people.
Failing that, actually scaring them to death would at least take them out of the equation.