Spectres can only be killed with potion damage. Apparently, this means they are completely immune to the /kill command.

One of my players accidentally left a spectre spawner running for too long, and I had to repeatedly teleport hundreds of spectres into a cloud of lingering harming for almost an hour, as there is no other way to kill them.

As an aside, I absolutely love this mod!! It brings so much life and variance to minecraft :)