If x grabs Y's hands without consent and invades y's personal space then y would be well within his right to knee X where he keeps his crown jewels! after all Y will be feeling threatened by X and has the right to defend himself! At the same time making a loud commotion of shouting go away, no, I do not want you near me, leave me alone, get your hands off me etc. would also help as it will draw a crowd and x will be more likely to be scared off.
bullies only target those who they think are able to be bullied, stand up to them and they often run in the opposite direction.
I was bullied for years at school, until the day I snapped and gave the bully a taste of her own medicine, she kept her distance after that.