Ex-friend being a pain in the hole to me.... wat can I do...she is REALLY nasty to me?

I was in the playground with my brother yesterday and she was there and she just looked at me and said to me ''ARE YA STARTIN ON ME NOW''...... and she said ill knock ya out ya *****... and then said ''that shut her up''..... i have no problem just ignoring her but what really gets to me is that she can threaten to KO me like that? WHAT THE BLOODY HELL????? i dont even talk to her anymore since 2months ... she acts really tough, has a bit of a rep for bein rough and she gets her brother to do the fighten for her. she 14, im 17... i regret bein involved wit her as friends because i dont act rough in my nature and im quiet but can give her a piece of my mind but i dont do street fighten (punchen)... what can I say to her? wat abut just tellin her to f *** uck off?