Minecraft – Hexxit – Rift Portal Problem

If you weren't in the Nether, then a simple solution would be to fill the volume completely with water. But that's not an option here since water cannot be placed in the Nether. Instead, we'll have to do what we can with solid blocks.

First of all, note that the spawn area is wide but not very thick: it is only 3 blocks high, centered on the spawner. So if you fill those three layers with blocks (the top and bottom layers could be slabs if you want the final result to be small, and the middle could be hollow), then you're done. In the picture you show, this would mean just adding more slabs (or full blocks) all around at the same level as the slabs you currently have on top of the spawners.

If you want to still be able to walk through the volume, I can think of two tricks:

• If you place a lower-half slab, and an upper-half slab two spaces above it, then the space between them is 2 blocks high so you can walk through it, and yet only the middle is actually empty air. Therefore, any monster 2 or more blocks high cannot spawn in the area; of the ones you list, only small magma cubes are small enough to spawn, and they're much less dangerous.

• Place a checkerboard pattern of wooden fences and nether brick fences. They won't connect, so there will only be single fenceposts and you can walk in the space between them. Cobblestone walls can also be used in this kind of way, but because they are thicker you can't move between two diagonally adjacent ones.

• Doesn't work: You could fill the volume with open fence gates, or doors. Or at least I thought you could — testing (version 1.8.7) shows that they now allow spawning.

The problem was with the overworld portal.

After wasted a few hours more trying to rebuild the portal on the nether side, I decided to rebuild the portal in the overworld "few blocks away", didn't count how many because didn't expect it to work, but it did.

Then I placed the portal back to where it was and the problem came back as expected.

So I

• Step by step removed all adjacent blocks from the portal and tried entering after every removed type - didn't work
• Cleared more then just adjacent block - didn't work
• Rebuilt the portal one block upwards didn't work
• Rebuilt the portal one block inwards - had high hopes on this one because "few blocks away" worked - but didn't work
• Rebuilt 2 blocks inwards - worked, but - only once, after the next try I caught on fire. Thought it was because I activated on the left bottom step and entered from the right but after numerous attempts didn't manage to get it working again

Disappointed that 2-blocks didn't work, I built it on the other side of my little room 9 blocks away and it worked! Sorry I couldn't bother to try what was the minimum amount of blocks from 2-9, but I guess because the problem didn't appear on one certain block, the minimum amount wouldn't be accurate anyways.