I don't think it's possible to delete or unattach a script, but you could use a http://www.creationkit.com/States_(Papyrus) to prevent any code from running once it's dead if you want.
If you've set it up to use events responsibly (so it's not receiving frequent updates when it doesn't need to be, for example) then you shouldn't need to worry about it.
I suppose the script won't cause too much trouble unless a dead actor gets hit... or has equipment taken off. I guess I'm just more concerned with all of the variables/properties taking up memory.
If you're worried about that, you might want to take a look at what the Creation Kit Wiki has to say about http://www.creationkit.com/Persistence_(Papyrus).