Make a new world. Build a pillar that is 25+ blocks high on the spawn point (preferably using a strong block so the player won't try breaking it) Go on and start building and stuff. When people want to join your game, but you don't want them to, they will spawn on the pillar and when they try to jump down, they will die. After awhile they'll leave cause they don't won't to keep dying :) I hope this helps! Also a good way to keep griefers from...griefing.