I found that there is a no_spawn.json and reference to creating a datapack for it, but I admit I am completely unfamiliar with how datapacks work, I'm used to config files.
I created a datapack in world/datapacks as block_spawn, with data/mob_grinding_utils/tags/entity_types/no_spawn.json containing the following. But this mob is still spawning. What am I doing wrong? Please help!

  1 {
  2   "values": [
  3     "enlightened_end:void_leviathan"
  4   ]
  5 }

no_spawn is for the spawner, use no_dreadful_spawn, assuming thats a hostile mob.

Oh! Ok. I didn't see any reference to a no_dreadful_spawn. Thanks!

As you can see in the debug screen here, I have the tag applied. But it still being spawned on dreadful dirt. If it matters, I'm on 1.20.1-1.0.3

in 1.0.3 the tag is "no_dirt_spawn", in 1.1.0 it was split into the dreadful and delightful variants.