If the "speaking softly" isn't working, it's time for the stick.
My aunt had an exchange student in her care. At some point there was an altercation and a boy at school slapped the (female) exchange student.
School didn't want to do anything about it, was hemming and hawing.
So my aunt phoned the boys mother and explained the situation - and was met with a "well I can't do anything".
My aunt simply replied "Well I can have her at the police station this afternoon and file a report, with photos of her bruises. After that I'm going to call my lawyer."
Amazingly, the parent of the troublesome student suddenly found the ability to do something!
Speaking softly only works of you're willing to use the stick.
If I squint my eyes I can sort of see what you're getting at. But on the flip side, this aunt also called the parent to make sure their duty was done. So it would be hard to claim "discipline your child, please, else I will need to file a police report to stop your child's harassment" is extortion.
Then police.
In some regions, this is a bit unrealistic.
I grew up in a neighborhood where the bully's dad was the police captain. The bully would target brown kids and property of their parents (e.g. M80 down the chimney, among other atrocities). Everyone in town new the son acted with impunity. Kids our age knew it was worse: the dad actively encouraged his son, and gave him the M80s. No one would talk about it in public, less they be targeted as well.
