I'm not sure if they have it for slugs yet but there are new products out for hornworms and some other critters, each with a specific bacteria that targets them but doesn't hurt anything else.
Otherwise... salt? Like a salt strip using tape they would have to slide across?
Salad is not as healthy for chickens as it is for humans 😋
If they get picky about real chicken feed, I would say to out stubborn them. But treats don't matter.