From my testing, it seems that by corrupting a single enemy's graphical data, two things occur:
- Terraria takes longer to load (staying in a black screen). This is permanent, until the original image files are put back in place.
- The enemy in question stops spawning completely.
I effectively removed blood crawlers and black recluses; enemies that are incredibly easy to find in their respective biomes, from my game. I stayed there for minutes and minutes and they simply didn't appear at all. In a spider nest, the monster count stayed at 4-7. When I reloaded the game with original graphical files of the black recluse, and stayed at the same location, the count quickly reached 20, as spiders started spawning, getting stuck somewhere, and attacking me.
Is anyone interested in this modification to the game? I warn you; it would be completely experimental. However, I can guarantee that this will cause no harm to your computer or the rest of your Terraria files.
I'm guessing that if Terraria can't read an enemy's sprite data, it refuses to spawn it to avoid possible glitches or crashes.