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.
2nd Event and Action would be pointless here. You are already testing against 'alive' variable in the first Event and the Playerhealth is only decreased if 'alive' is true - no need to test for false.