It was recently fixed so that the banshee's wail doesn't effect animated trees. In the past, various powerful NPCs (like orb guardians) were similarly made immune to the banshee to stop abuses. It's still possible (I think) to clear out the bug temple using wished-for banshee. In the future, maybe some other abuses will be found and require fixing. Will the monsters of the volcano need to be immune to the banshee as well, for instance?

Rather than making special case after special case, why not just address the problem at the source by making it so the banshee can never appear outside of her level. This would require:

1. Making it so the bashee doesn't follow the PC up or down stairs.
2. Making it so banshees cannot be wished for (could result in ghost lords instead, or just hearing the scream in your head, or even just nothing happening).
3. Making sure that nothing the player can use to summon monsters (spell, scroll, wand of monster creation, figurine, etc.) could ever generate a banshee. This may already be the case.
4. (Optional.) Making sure that no monsters who can summon could randomly summon a banshee. I've never seen this happen myelf, but I vaguely recall some poor unfortunate reporting it on rgra many years ago.

I understand if you'd prefer to keep this in the game as an imaginative solution to a problem, but personally, I'd prefer to see it gone.