I have two objects troll and pig. I'm detecting collisions between these two characters with Pygame's colliderect function. collisions for this works perfectly, however I am unable to remove the object from the screen when a collision occurs. When a troll collideswith a pig, I want the pig to disappear from the screen. I set an image for both of these and bit them, is there a way to un-blit? Ive tried pygame.image.set_alpha(255) (which set full transparency of an image) function to no avail. 
Any suggestions?
Can't you just stop drawing the pig? Each frame you fill the screen with the background, so it won't be visible.
remove object after collision