" ... a recent OSU university study [of online bullying found] ... only 10 percent of the students who noticed the abuse directly intervened, either by confronting the bully online or helping the victim." - John Biggs, "Study: Most People Won't Stop Online Bullies," TechCrunch, February 26, 2015. Here is the article.
That's the bad news. The good news is that about 70 percent of those who didn't directly intervene later reported the bully, when given the opportunity.
For those with the resources, the solution may be to activate third parties to take on the bullies. That would be most effective if those in the loop bully the bullies. We know from experience and research bullies cave when there is push-back.