How do I keep the player from automatically changing direction?

The WASD controls in the ThirdPersonController script are based on the main camera orientation: W goes in the camera's forward direction, D goes to its right direction, and so on - that's why the character changes direction when a new camera is selected.
Changing this would require extensive modifications in the original script - I suggest you to remove the scripts ThirdPersonController and ThirdPersonCamera and use a custom script to control both, the character and the camera.
The script below does basically what you want, and may serve as a starting point. It doesn't use multiple cameras - instead of this, the main camera is translated instantly to the pre-set positions when their associated triggers are entered. These positions are defined by empty objects tagged as "CameraPos", which contain only a box (or sphere) collider adjusted to the desired area.
That's the character script:

 var walkSpeed: float = 4.0;
 var runSpeed: float = 8.0;
 var jumpSpeed: float = 8.0;
 var turnSpeed: float = 90.0;
 // animation names
 var walk = "walk";
 var run = "run";
 var idle = "idle";
 var jump = "jump_pose";
 var walkLim: float = 0.2; // enters walk animation above this speed
 var runLim: float = 5.0; // enters run animation above this speed
 
 private var velY: float = 0.0; // current vertical velocity (used for gravity)
 private var gravity: float = 10.0;
 private var jumping = false;
 private var controller: CharacterController;
 private var trCam: Transform;
 
 function Start(){
   controller = GetComponent(CharacterController);
   trCam = Camera.main.transform;
   animation[jump].wrapMode = WrapMode.ClampForever;
 }
 
 function Update(){
   // movement control: AD rotates, WS goes forth/back
   transform.Rotate(0, Input.GetAxis("Horizontal") * turnSpeed * Time.deltaTime, 0);
   var speed = walkSpeed; // select moving speed
   if (Input.GetKey("left shift") || Input.GetKey("right shift")){
     speed = runSpeed; // shift activates sprint
   }
   var moveVel = transform.forward * Input.GetAxis("Vertical") * speed;
   if (controller.isGrounded){
     jumping = false; // character landed: isn't jumping anymore
     velY = 0;
     if (Input.GetButton("Jump")){
       velY = jumpSpeed;
       jumping = true; // character started the jump
     }
   }
   velY -= gravity * Time.deltaTime; // calculate gravity effect
   moveVel.y = velY; // add it to the current movement velocity
   controller.Move(moveVel * Time.deltaTime); // move the character
   // animation
   var vel = controller.velocity.magnitude; // base it on current velocity
   if (jumping){ // jumping is prioritary!
     animation.CrossFade(jump);
   } else if (vel > runLim){ // if not jumping, check run...
     animation.CrossFade(run);
   } else if (vel > walkLim){ // else check walk...
     animation.CrossFade(walk);
   } else { // else go to idle
     animation.CrossFade(idle);
   }
 }

 // camera control

 function LateUpdate(){
   trCam.LookAt(transform); // always look at the character
 }
 
 function OnTriggerEnter(other: Collider){
   if (other.tag == "CameraPos"){ // if entered a CameraPos trigger...
     trCam.position = other.transform.position; // move camera to its position
   }
 }