如果敌人的rect不在屏幕内，他们可以通过调用self.kill()将自己从游戏中移除，例如：

def update(self):
    if self.rect.colliderect(self.game.player.rect):
        self.game.restart() # reset score + count deaths

    # we can simply use move_ip here to move the rect
    self.rect.move_ip(0, (self.game.score + 500) / 100)

    # check if we are outside the screen
    if not self.game.screen.get_rect().contains(self.rect):
        self.kill()

另外，你也可以使用pygame的事件系统来代替线程来生成敌人。下面是一个简单的，不完整的，但可以运行的例子（注意注解）：

import random

import pygame

TILESIZE = 32
WIDTH, HEIGHT = 800, 600

# a lot of colors a already defined in pygame
ENEMY_COLOR = pygame.color.THECOLORS['red']
PLAYER_COLOR = pygame.color.THECOLORS['yellow']

# this is the event we'll use for spawning new enemies
SPAWN = pygame.USEREVENT + 1

class Enemy(pygame.sprite.Sprite):

    def __init__(self, game):

        # we can use multiple groups at once. 
        # for now, we actually don't need to
        # but we could do the collision handling with pygame.sprite.groupcollide()
        # to check collisions between the enemies and the playerg group
        pygame.sprite.Sprite.__init__(self, game.enemies, game.all)
        self.game = game
        self.image = pygame.Surface((TILESIZE + 10, TILESIZE + 10))
        self.image.fill(ENEMY_COLOR)

        # we can use named arguments to directly set some values of the rect
        self.rect = self.image.get_rect(x=random.uniform(0, WIDTH - TILESIZE))
        # we dont need self.x and self.y, since we have self.rect already
        # which is used by pygame to get the position of a sprite

    def update(self):
        if self.rect.colliderect(self.game.player.rect):
            self.game.restart() # reset score + count deaths

        # we can simply use move_ip here to move the rect
        self.rect.move_ip(0, (self.game.score + 500) / 100)

        # check if we are outside the screen
        if not self.game.screen.get_rect().contains(self.rect):
            self.kill()

class Player(pygame.sprite.Sprite):

    def __init__(self, game):
        pygame.sprite.Sprite.__init__(self, game.all, game.playerg)
        self.game = game
        self.image = pygame.Surface((TILESIZE, TILESIZE))
        self.image.fill(PLAYER_COLOR)
        self.rect = self.image.get_rect(x=WIDTH/2 - TILESIZE/2, y=HEIGHT-TILESIZE*2)

    def update(self):
        # no nothing for now
        pass

class Game(object):

    def __init__(self):
        # for now, we actually don't need mujtiple groups
        # but we could do the collision handling with pygame.sprite.groupcollide()
        # to check collisions between the enemies and the playerg group
        self.enemies = pygame.sprite.Group()
        self.all = pygame.sprite.Group()
        self.playerg = pygame.sprite.GroupSingle()
        self.running = True
        self.score = 0
        self.deaths = -1
        self.clock = pygame.time.Clock()

    def restart(self):
        # here we set the timer to create the SPAWN event
        # every 1000 - self.score * 2 milliseconds
        pygame.time.set_timer(SPAWN, 1000 - self.score * 2)
        self.score = 0
        self.deaths += 1

    def run(self):
        self.restart()
        self.player = Player(self)
        self.screen = pygame.display.set_mode((WIDTH, HEIGHT))

        while self.running:
            for e in pygame.event.get():
                if e.type == pygame.QUIT:
                    self.running = False
                # when the SPAWN event is fired, we create a new enemy
                if e.type == SPAWN:
                    Enemy(self)

            # draw and update everything
            self.screen.fill(pygame.color.THECOLORS['grey'])
            self.all.draw(self.screen)
            self.all.update()
            pygame.display.flip()

            self.clock.tick(40)

if __name__ == '__main__':
    Game().run()

检查self.rect.y的值，WIDTH，也许是kill方法。看起来self.rect.y总是大于或等于WIDTH，这就是为什么它把它们都杀死了。