Use a command block to "Sense" a player in a certain area

On my friends and my server, we have a skyblock map. My issue is when somebody joins they spawn above the Void. What I need is for a command block to determine if a player is near the Void and send a redstone signal to another command block that will teleport them to the main island.

You can make a redstone clock (I recommend the repeater ones) and attach it to a command block with this command:

/spawnpoint @a X Y Z

Remember to turn off command block output to prevent spam!

/gamerule doCommandBlockOutput false

Hope it helped. :)

What you need is if they "spawn" above void then go to the spot you want to be the spawn point and type in the chat bar /setworldspawn

This will set the worlds spawn to that spot instead of trying to have lots of command blocks

:)

Why not just get the Essentials plugin and set the spawn somewhere else? You can use the /setspawn command. This is, of course, assuming you're using Bukkit.