How do I make a game object freeze in the y axis when hitting a collider?

 #pragma strict
 var Ggroup : GameObject[];
 var group: GameObject[];
 var moveSpeed : float = 0;
 var speed : float = 6;
 
 function Start () 
 {
     Physics.IgnoreLayerCollision(8,9,true);
     Ggroup = GameObject.FindGameObjectsWithTag("Enemy1");
     for(var i=0;i<6;i++)
         {
             for each (E in Ggroup)
                 {
                     if (Vector3.Distance(E.transform.position,transform.position)<150)
                     {
                         group[i]=E;
                         break;        
                     }
                 }
         }
 }
 
 function Update () 
 {
     for (var x=0;x<group.Length;x++)
     {
         if(Vector3.Distance(group[x].transform.position,transform.position)<3)
             {
                 Debug.Log("avoid");
                 var delta = group[x].transform.position - transform.position;
                 delta.Normalize();
                 delta = Vector3.one-delta;
                 transform.position = transform.position + (delta * moveSpeed);
             }
     }
     
 }

This is the script I gave to enemies that spawn so that they avoid each other (part of a flocking algorithm), the problem is that sometimes to avoid each other they jump in the y axis rather than moving away in the horizontal axes of the scene. I think the best way to avoid this would be to freeze them in the y axis once they hit the collider of the terrain (they spawn higher up and I don't want them to freeze in the y axis on spawn), is it possible for me to do it? What would I need to do?