this is the best approach honestly. redirect them to some place that undermines their efforts. either back to themselves, their own provider, or nasty crap that no one want to find in their crawler logs.
Decades later, I'm still traumatized by goatse, so it'll have to be someone with more fortitude than me.
mind you before google and the likes and the great purge of internet, these things were mild and humorous...
Wouldn't recommend Googling it. You either know or just take a guess.
The first goatse I actually saw was in ASCII form, funnily enough.