Well, you just have to disable the part which makes the ennemy affect the player's health. I presume it's like "on collision with player" => player health = player.health - 1 or something. You can make a boolean variable "alive" initialised on true, and when this variable is false (after the ennemy's deafeat), then the ennemy doesn't affect the health anymore. For example :
[event]
Ennemy : On collision with Player
variable alive = true
[action]
playerHealth = playerHealth - 1
[event]
Ennemy : On collision with Player
variable alive = false
[action]
playerHealth = playerHealth
It's an idea, I didn't tested it, but that's how I'll try it.