In vanilla Minecraft, is it possible to block creation of new Nether portals? This is for a Survival/Adventure hybrid map; I want the player to have a great deal of freedom, but to control access to the Nether to portals I've placed in the world. I can ask players to not build new portals, but it would be nice to have the rule mechanically enforced.
Possibilities include: somehow detecting and destroying the Nether portal blocks when they appear; not allowing the portal to be activated in the first place, and damaging the obsidian frame; somehow "tainting" any obsidian in the world or created so it's no longer eligible to be part of a port.
I'm at a loss for what a solution might look like beyond "Probably some command blocks." Trying to /testforblock every block in the world seems impractical.