How to prevent movement in a specific direction only?

I have a trigger box and when the player is in this trigger I would like to prevent their movement in a specific direction. For example they can move forward, backward and left but not right. I have previously controlled movement along a specific axis by setting the rigidbody.velocity.x/rigidbody.velocity.z to zero; however in this case I do not want the entire axis movement stopped, only in a specific direction. Since I am using a rigidbody/capsule collider setup, I have tried experimenting with the position constraints however these do not completely stop the player from moving. What is an effective way to do this?

Here is the code that I am using:

          //Cover Left Stop Movement
         if (Input.GetButton("Left") && inCoverLeft) {
             stopMovement = true;
         }
         else {
             stopMovement = false;
         }
         //Cover Right Stop Movement
         if (Input.GetButton("Right") && inCoverRight) {
             stopMovement = true;
         }
         else {
             stopMovement = false;
         }

While the player stays in a left or right trigger the relevant boolean (inCoverLeft or inCoverRight) is set to true. I then check in update for when the boolean is true and either the left or right arrow is being pressed. If both the relevant boolean and the corresponding arrow are "true" I set another boolean (stopMovement) to true which is checked for when applying movement physics to the player. Now, while this does stop the player initially, the problem is that once stopped if you continue to quickly tap the arrow key it slowly but surely nudges the player inside the trigger. This is because the movement is stopped only when the key is being held and therefore in the split second before the stopMovement boolean is set to true, the psychics movement gives the player a small nudge each time.