I took a quick look at your /visit. For the most part you do have enough light to stop the spawning, however I did find some spots where the light level was low enough for them to spawn. Also since you have stairs going down there is low light in the lower levels so they can spawn there. Also in the room with your beacon, there is very low light so they will spawn there.
Your light level needs to be at least 8 or higher. Any level 7 or below will allow zombies to spawn.
Hope this helps.