Unscheduled off dimension activation! Close the iris! *WHAM!* ... That was close. ;D
You could construct a water-activated "iris" next to any portal, using sand blocks, torches and controllable water flow. It could work like its archetype - anything teleporting in will be stuck inside the sand and suffocate.
Looks pretty cool, although you may have a ghast problem with the first two. I'm going to be eliminating my tunnel systems and replacing them with portals much like you've done to link my various buildings/mines.
has anyone else thought about putting double steel doors in front of the gate? saw the Idea on one of the other threads And I think I will implement it in my game. my gate is still in a small (less then 5x5x5) secure room.