It's relatively easy, especially if you live near an ocean, to make an enderman farm teleport proof. Just build the farm out about 32 blocks out from shore, rise it up another 32 blocks from ocean surface, and cover the roof of the farm with water They won't be able to teleport anywhere... You can even cover the ground they spawn above with a water current to carry them to the turtles, so that they're killed a little bit faster. The same can be done out of the ocean, however it's advised to build the farm higher up in the sky, since they can teleport farther away on the Y axis than they can on the X and Z axis. Basically? As long as there isn't a block that isn't either air or covered in water in a 32 block radius (x/z) around where they're spawning, they'll not teleport anywhere, they'll just sit still, even when taking damage from water... Which is funny when done in a Mystcraft age with the Rain symbol, since you can build a farm with no roof or walls, and a floor made of transposers, and the endermen will just die from the rain where they stand.