This plugin was created with the intention of removing the AI of mobs from spawners, but only those specified in the configuration, since the function that is implemented in spigot automatically nerf all mobs without a choice. Nerfed mobs can still be pushed by water, hits, etc.

Simple configuration, just list the mob types you want to nerf on config.yml
All entities: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/entity/EntityType.html