Are you trying to do damage to a player or remove a prop? Those are two different things. Maybe post what you have so far so we can help you.
As for removing an entity after it does something, you can try 2 things:
1. Create a logic_timer or output to delete it after some delay time.
2. Just make the first line of your script: ent_fire <classname|targetname> kill. This will remove the previous entity each time you run the script.