The spider den decoration disappears, after any spider got a hit from anything inside the area of decorated spider den. You need to decorate your den every time when a spider around the den got hit from hounds, frogs or mating beefalos, making spiders attack walls around the nest constantly after the deco disappears.
1) Decorate the spider den.
2) Hit the spider around the spider den.
3) Decoration disappears.