I hate to necro, but I am having the same problem and it appears there is still no solution. I have found that using Worldguard provides limited help in staving off ZPN. You can make the tunnel a polygonal region that does not allow ZPN spawning. Issues with this idea are:

• the original poster may not wish to use a plugin such as Worldguard
• if the track has several turns, it can be difficult to make it into a single region, creating a mess of regions to cover the whole track
• this is not a vanilla Minecraft method, so it is sort of an "exploit" or "cheating"

I am having the same problem and it appears there is still no solution. I have found that using Worldguard provides limited help in staving off ZPN. You can make the tunnel a polygonal region that does not allow ZPN spawning. Issues with this idea are:

• the original poster may not wish to use a plugin such as Worldguard
• if the track has several turns, it can be difficult to make it into a single region, creating a mess of regions to cover the whole track
• this is not a vanilla Minecraft method, so it is sort of an "exploit" or "cheating"