Stop object movement in 1 direction along axis

I am working on making part of a puzzle. I want the player to only be able to walk down a hallway when facing backwards.

1. There is a hallway

2. There is a collider set as a trigger to detect when player is in hallway

3. On the player I have a script that detects Y rotation

4. When the player is in the hallway and is facing forwards they should not be able to move up the Z axis only down

I have everything working properly, I just don't know how to stop the object from moving up the Z axis but allow it to move down.

 var inHall : boolean = false;
 var canWalk : boolean = false;
 var yRotation : float;
 
 function Start () 
 {
 
 }
 
 function Update () 
 {
     var currentPositionZ = this.transform.position.z;
     var lastPositionZ : float;
     yRotation = this.gameObject.transform.rotation.eulerAngles.y;
     
     if(inHall)
     {
         if(this.gameObject.transform.rotation.eulerAngles.y <= 240 && this.gameObject.transform.rotation.eulerAngles.y >= 120)
         {
             canWalk = true;
         }
         else
         {
             canWalk = false;
         }
     }
     else
     {
         canWalk = false;
     }
     
     if(!canWalk)
     {
         if(currentPositionZ >= lastPositionZ)
         {
             currentPositionZ = lastPositionZ;
         }
     }
     
     
     lastPositionZ = currentPositionZ;
 }
 
 function OnTriggerStay (col : Collider)
 {
     if(col.tag == "Hall")
     {
         inHall = true;
     }
 }
 function OnTriggerExit (col : Collider)
 {
     if(col.tag == "Hall")
     {
         inHall = false;
     }
 }

This is the full code.

The problem area is:

 var currentPositionZ = this.transform.position.z;
 var lastPositionZ : float;
 
 if(!canWalk)
 {
     if(currentPositionZ >= lastPositionZ)
     {
         currentPositionZ = lastPositionZ;
     }
 }
     
     
 lastPositionZ = currentPositionZ;

This method of returning the Z coordinate to its previous value if it moves up does not work. I need help because i dont know how to do this. Thank you