How to do be popular in elementary school etc?

im audrey's little sister brittney and im in 5th grade and there is this girl in my cl named kelsey and shes taking over my friend isabel and everytime i ask her to play with them kelsey says that their having a day of "theirselves" and it happens everytime what should i do for payback and dont say to be nice because that never works and i always have noone to play with and isabel doesnt like kelsey that much either so how do i make kelsey have no one to play with and i dont take " be nice" as an anwser cuz i hate her and kelsey is fugly and a b**** i know im not supposed to swear but im just so frikin mad at her.