To be honest, the moment Dia made a miraculous showing against the proto-Dark Eldar, we more or less had little to no endgame against this particular Eldar. She has no sense of mercy, and once attention is attracted there's little hope of her getting bored and wandering off without us being squashed. (Dark) Eldar are dicks like that - they like to destroy toys once they're bored with them.
* She *might* be able to be chased off by the Exodites, but they're not particularly interested in doing so and she knows it.
* There's unlikely to be a 'bigger fish' to devour her short of an existing Greater Daemon getting free on Dandriss. Another Eldar out for something interesting might fight and kill her, but that's also unlikely. In her search for entertainment, she *might* manage to have an accident with either the Warp or some form of super-Dandriss wildlife that hard-counters Eldar. I'm not going to *quite* pray for an opportunistic Enslaver, but...
* As we just demonstrated, she seems to be quite capable of being independent of biology for survival. We can try tearing her apart with gravity or some kind of annihilation (anti-matter or the like) in hopes of giving her nothing to anchor her to the Materium, and perhaps there's some way to trip her into the Warp in some way (... yeah, no, not gonna happen).
* Her telepathic and divinatory abilities means that even having a method to successfully take her out is going to be
very hit or miss.
* There's few other things that would attract her attention enough to ignore us. The Authority melting down spectacularly
might do it, albeit leaving us with another problem. But there's unlikely to be anything else more interesting then Dia and her family on the radar.
* Waiting her out is hazardous, and leads to her deciding to wipe the family out before she goes off somewhere else nine times out of ten.
* Attempting to stay interesting for generations is... well, I don't like it, and that's a long time to play that game with little margin for error.
* We can attempt to buy time to diversify the family, so if she decides to destroy her toys and move on to some other planet the dynasty can continue.
* We can take the Necron Pylon approach and try and find a way to successfully take her psychic powers out of the equation in an encounter. Yeah... I didn't think so either.
* Eldar *do* have a finite life span, albeit one measured in millenia rather then decades. We could hope she's near the end of hers, but that doesn't really help much.
* The long life-span does mean that we might not need to face her games more then once or twice a generation? That's a horrible silver lining >_<
Bleh. AN really outdid himself in finding a persistent villain that we're unlikely to be able to dispose of for the entirety of the quest. I've been kind of aware of that ever since the Eldar were introduced, but have been trying to not think of it.