OnSpawningEntity

[xoft]

Use the OnSpawningEntity() hook and return true if you don't want the entity to spawn.
Also, as Bearbin said, use named constants for biomes: biForest, biTundra etc. Here's a list:

biOcean            = 0,
biPlains           = 1,
biDesert           = 2,
biExtremeHills     = 3,
biForest           = 4,
biTaiga            = 5,
biSwampland        = 6,
biRiver            = 7,
biHell             = 8,  // same as Nether
biNether           = 8,
biSky              = 9,
biFrozenOcean      = 10,
biFrozenRiver      = 11,
biIcePlains        = 12,
biTundra           = 12,  // same as Ice Plains
biIceMountains     = 13,
biMushroomIsland   = 14,
biMushroomShore    = 15,
biBeach            = 16,
biDesertHills      = 17,
biForestHills      = 18,
biTaigaHills       = 19,
biExtremeHillsEdge = 20,
biJungle           = 21,
biJungleHills      = 22,