How to Stop Nether Portal Spread Around a Specific Portal

The Coal Block Method

The mod's built-in way to halt transformation around a single portal is to place four coal blocks within the portal's spread radius. Once all four are present, the Nether conversion for that portal stops. This is the intended in-world solution and does not require touching the config.

1. Stand at the portal and note the spread radius (default is 15 blocks, but check your config since it may differ).

2. Place four coal blocks anywhere inside that radius — they do not need to form a specific shape or touch the portal frame.

3. If it continues, one of your coal blocks is likely just outside the radius.

Things to Watch For

• Coal blocks must be actual placed blocks, not items in a chest or dropped on the ground.
• If you break a coal block, the spread resumes on its next tick.
• The radius is measured from the portal, so on a large portal you may want the coal blocks spread around the perimeter rather than clustered on one side.
• This only affects the portal you placed them near. Other portals in your world keep spreading unless they also have four coal blocks.

If you want to disable spread globally instead of per-portal, that is a config change, not an in-game action.