On and off teleporters Blocked Teleporter at the beginning of the game: First build a tower where the teleporter will be and put some explosives at the top. Set those explosives to not spawn at the beginning and the the respawn time for how long you want the teleporter to be blocked. Delete the tower and place a teleporter under the explosives. Then block the teleporter with explosives. Set the new explosives to spawn only at the start and not respawn. There you have it, they can be unblocked by people if they are accesible, so put them where people can't see them if you want. To have them turn on and off in the game, set the blocking explosives to respawn. Set the tower explosives to be placed at start if you want it first on or set them to not spawn at start to have the teleporter start blocked.
The best version I can see of it would be using a grav lift like the above poster said, but can you blow up a grav lift very easily with a falling fusion core?