I'm using the mod pack MC Eternal found with the twitch launcher.
I've made a base really high in the mountains and we have a MASSIVE infestation of harpies, I've tried using mob repelling blocks, turrets from open turrets and immersive engineering, NOTHING targets harpies.

they are completely invisible to the turrets, I've tested this with skeleton Warriors, and they can be targeted, I'm unsure on the issue, but it is extreemly annoying, I've also lit the entire mountain top up and they still spawn (although I might have missed some dark areas.

Harpies don't appear to respect light levels at all, they just spawn when it's night time. The only thing that works is blocks mobs can't spawn on, like half slabs.

OR you can add the mod TorchMaster to the pack, if you're able to, then set the more aggressive spawn blocking to true in TorchMaster's config, and then Mega Torches will stop their spawning.