How can I completely disable a particular vanilla mob in Minecraft 1.7.10?

Solution 1:

I actually found a solution that works for me, specifically. The More Commands mod includes the /ignorespawn command, which prevents mobs from ever spawning. They will still appear in spawners, but only the little model shows; they don't actually spawn. The model spins, and periodically the spawner flashes, but no spiders. I still need to ask my friend, but I don't think that will be a problem; he will probably just be very motivated to break those spawners. :-)

http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2382164-more-commands-v2-4-adds-100-commands-to-the-chat

Not quite as good as removing them entirely, but I think it will work, and it is an easy way to avoid actually encountering mobs you don't want to see. You can add the command in the configuration, and it will be active on all world's by default.