In order to find myself some nice lapis lazuli and get away from my current base, I'm building a little tunnel in the Nether to allow me to get into the midst of some fresh chunks fairly quickly.
I have a nice little stone house, but as Ghasts and zombie pigmen can spawn at "any light level" in the Nether, how do I keep them from popping up in my Nether regions where I don't want them? Will they even spawn on "foreign" blocks (e.g. those that aren't Netherrack, slow sand, etc.)
As of the Halloween Update:
You don't have to worry about Zombie Pigmen (they are, after all, neutral mobs until provoked).
Ghasts can safely be kept out by restricting yourself to not using any of the 4x4x4 (or larger!) areas that they need to spawn. Keep a tight ceiling, and you'll be fine.
Hehe, Withers and Blazes only spawn in certain locations, namely Nether Forts and between a height of 64-74 (ish, I do not have the exact values with me). The easiest way to get them to not spawn is to simply move out of the nether fort that you have set up shop in. The pigmen will be and always are a problem but if left alone they are just annoying.
If you are set on being in a nether fort though, then I suggest half slabs. Nothing likes spawning on top of half slabs regardless of how much space overhead you need.. Also keep in mind a Ghast loves to spawn in a 5x5x5 area. Having one of them appear in your house really really sucks.
Ghasts are the real problem; if you want to have large rooms, build pillars or let some blocks float in the air (not very handy). You can also make a trap with some dispensers to get some pork and gunpowder.
I build up. To the top of the Nether. There is lots of Netherrack up there that you can dig into. All my Nether tunnels are at the point that their ceiling is Bedrock. I can travel with tunnels completely uninterrupted for as far as I want to. I still play on my original world from Pre 1.0 so I tend to do a lot of Nether travelling to get to new chunks.
I did initially, after my very first portal, have to build a staircase/ladder system to get to the very top that's surrounded by that nicely strong cobblestone, but after that I didn't have to bother with Ghasts or any mobs until I decided to explore the Nether for all its secret goodies.
I'm not sure if it's less likely for Pigmen to spawn at those heights but I rarely see them spawn in my tunnels, not that I have to worry about them. The tunnels are also, well tunnels, so no room for Ghasts to spawn. Once you're up there you can place a portal and never have to go back down until you need to.
To keep out Ghasts I make the ceiling low (I can barely walk in my house!), only one floor and a really, really tiny panic room just in case a ghast decided to screw the spawn limits.
If you have a problem with pigmen, you could make iron golems in the Nether to protect you. To make sure they don't fall in lava put walls around the cliff edges. If any blazes are flying around your area make water walls and they will die.
