Use the pause button. Its nearly impossible for someone to get kicked if the game is paused when they join. However, sometimes pause will only prevent the player to get kicked while paused. So in those cases, change/restart map to ensure the player can stay. When the game gets heavy lag, pause might help too. Don't ruin the game by holding pause for minutes, though. A second or 3 should be enough, unless the lag hasn't stopped yet then.