What to do about 9yr Harment?

This 9yr got all mad at alittle "Joke" and told her mom, now her mom says we can't be her friends and I said Ok and walked away. Then the girls started fake laughing, trying to make us jealous, and then she brought out her friend and her sister, and they started throwing wrappers and a toy ball and a pool noodle in our yard, so I just took them and threw them away. They started calling us "Hobknockers" and "Aliens" and whenever I even LOOKED at them they say "Boo" and just wont stop. I came inside to get away from it and I came back outside when it was dark, and they had knocked down our chairs, and they had door-ditched us like 5 times. It's getting annoying and I don't know what to do! Any advice? Not sure if it's actually harment or not.