AI scripting in Unity. Blocking Y rotation in code - unity3d

Question

I am creating a game where the world is 3D yet the characters are 2D. This means that my Y rotation must not change or else the 2D sprite rotates. I have a been scripting an AI and while the enemy does attract towards my player, he also rotates by 90 degrees. How can I modify my code to make it so that the sprite follows my player in the X and Z axis but the Y axis rotation stays zero.
//CODE STARTS
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class AISimple : MonoBehaviour
{
public Transform player;
float distancefrom_player;
public float look_range = 20.0f;
public float agro_range = 10.0f;
public float move_speed = 5.0f;
public float damping = 6.0f;
// Use this for initialization
void Start()
{
}
// Update is called once per frame
void Update()
{
distancefrom_player = Vector3.Distance(player.position, transform.position);
if (distancefrom_player < look_range)
{
lookAt();
}
if (distancefrom_player < agro_range)
{
attack();
}
}
void lookAt()
{
Quaternion rotation = Quaternion.LookRotation(player.position - transform.position);
transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * damping);
}
void attack()
{
transform.Translate(Vector3.forward * move_speed * Time.deltaTime);
}
}

Answer 1

rigidbody has this functionality, if you tick freeze y rotation you'll be fine

Answer 2

okay, then try to modify y rotation before you set it
void lookAt()
{
Quaternion rotation = Quaternion.LookRotation(player.position - transform.position);
var euler = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * damping).eulerAngles;
euler.y = 0f;
transform.rotation = Quaternion.Euler(euler);
}