Prevent an IMGUI button from passing throught to the ground on a point and click game using nav mesh

Hi, to who would be interested in helping me.

I have this script that is attached to a game object so when the player enters the trigger zone it fires up an IMGUI button that the player can click on to go to a menu to purchase a selection of mercenaries.
My problem is every time I click on the IMGUI button the menu comes up but does not stay up because the click passes through to the ground and moves the character forward.

Here my Script.

using UnityEngine;
using System.Collections;

public class HireMercenaryC : MonoBehaviour
{

public bool startAfterTalk = false;
private bool begin = false;
private bool noCash = false;

private GameObject player;
private bool enter = false;

[System.Serializable]
public class MercenaryInfo
{
public string className = "";
public GameObject mercenaryPrefab;
public int level = 1;
public int atk = 1;
public int def = 1;
public int matk = 1;
public int mdef = 1;
public int hp = 200;
public int price = 500;
}

public MercenaryInfo[] mercenariesInfo = new MercenaryInfo[9];

void Update()
{
if (Input.GetKeyDown("e") && enter && !begin && !startAfterTalk)
{
begin = true;
}
if (startAfterTalk && enter && !begin)
{
begin = GetComponent<DialogueC>().talkFinish;
}
if (startAfterTalk && begin)
{
GetComponent<DialogueC>().talkFinish = false;
}
}

void OnGUI()
{
if (begin && !noCash && player)
{
GUI.Box(new Rect(Screen.width / 2 - 320, 120, 680, 400), "Mercenaries List");
if (GUI.Button(new Rect(Screen.width / 2 + 160, 460, 120, 50), "Cancel"))
{
CloseWindow();
}
GUI.Label(new Rect(Screen.width / 2 - 290, 490, 200, 30), "$ " + player.GetComponent<InventoryC>().cash);
//---------------------------------------
if (GUI.Button(new Rect(Screen.width / 2 - 285, 140, 200, 90), mercenariesInfo[0].className + " : LV " + mercenariesInfo[0].level + "\n Pay :" + mercenariesInfo[0].price))
{
GetMercenary(0);
CloseWindow();
}
if (GUI.Button(new Rect(Screen.width / 2 - 75, 140, 200, 90), mercenariesInfo[1].className + " : LV " + mercenariesInfo[1].level + "\n Pay :" + mercenariesInfo[1].price))
{
GetMercenary(1);
CloseWindow();
}
if (GUI.Button(new Rect(Screen.width / 2 + 135, 140, 200, 90), mercenariesInfo[2].className + " : LV " + mercenariesInfo[2].level + "\n Pay :" + mercenariesInfo[2].price))
{
GetMercenary(2);
CloseWindow();
}
//----------------------
if (GUI.Button(new Rect(Screen.width / 2 - 285, 250, 200, 90), mercenariesInfo[3].className + " : LV " + mercenariesInfo[3].level + "\n Pay :" + mercenariesInfo[3].price))
{
GetMercenary(3);
CloseWindow();
}
if (GUI.Button(new Rect(Screen.width / 2 - 75, 250, 200, 90), mercenariesInfo[4].className + " : LV " + mercenariesInfo[4].level + "\n Pay :" + mercenariesInfo[4].price))
{
GetMercenary(4);
CloseWindow();
}
if (GUI.Button(new Rect(Screen.width / 2 + 135, 250, 200, 90), mercenariesInfo[5].className + " : LV " + mercenariesInfo[5].level + "\n Pay :" + mercenariesInfo[5].price))
{
GetMercenary(5);
CloseWindow();
}
//--------------------------
if (GUI.Button(new Rect(Screen.width / 2 - 285, 360, 200, 90), mercenariesInfo[6].className + " : LV " + mercenariesInfo[6].level + "\n Pay :" + mercenariesInfo[6].price))
{
GetMercenary(6);
CloseWindow();
}
if (GUI.Button(new Rect(Screen.width / 2 - 75, 360, 200, 90), mercenariesInfo[7].className + " : LV " + mercenariesInfo[7].level + "\n Pay :" + mercenariesInfo[7].price))
{
GetMercenary(7);
CloseWindow();
}
if (GUI.Button(new Rect(Screen.width / 2 + 135, 360, 200, 90), mercenariesInfo[8].className + " : LV " + mercenariesInfo[8].level + "\n Pay :" + mercenariesInfo[8].price))
{
GetMercenary(8);
CloseWindow();
}
}

if (noCash)
{
GUI.Box(new Rect(Screen.width / 2 - 125, 220, 250, 120), "Not Enough Cash!!");
if (GUI.Button(new Rect(Screen.width / 2 - 75, 255, 150, 60), "OK"))
{
noCash = false;
}
}

}

void OnTriggerEnter(Collider other)
{
if (other.tag == "Player")
{
player = other.gameObject;
enter = true;
begin = false;
}
}

void OnTriggerExit(Collider other)
{
if (other.tag == "Player")
{
enter = false;
CloseWindow();
}
}

public void CloseWindow()
{
if (startAfterTalk)
{
GetComponent<DialogueC>().talkFinish = false;
}
//enter = false;
begin = false;
}

public void GetMercenary(int id)
{
if (player.GetComponent<InventoryC>().cash >= mercenariesInfo[id].price)
{
//Get You Mercenary.
player.GetComponent<InventoryC>().cash -= mercenariesInfo[id].price;
if (!player.GetComponent<SpawnPartnerC>())
{
player.AddComponent<SpawnPartnerC>();
}
if (mercenariesInfo[id].mercenaryPrefab)
{
//Spawn Mercenary
Vector3 pos = player.transform.position;
pos.y += 0.2f;
pos.x += Random.Range(-1.5f, 1.5f);
pos.z += Random.Range(-1.5f, 1.5f);

GameObject m = (GameObject)Instantiate(mercenariesInfo[id].mercenaryPrefab, pos, player.transform.rotation);
m.GetComponent<AIfriendC>().master = player.transform;
//Apply Mercenary's Status
m.GetComponent<StatusC>().level = mercenariesInfo[id].level;
m.GetComponent<StatusC>().atk = mercenariesInfo[id].atk;
m.GetComponent<StatusC>().def = mercenariesInfo[id].def;
m.GetComponent<StatusC>().matk = mercenariesInfo[id].matk;
m.GetComponent<StatusC>().mdef = mercenariesInfo[id].mdef;
m.GetComponent<StatusC>().maxHealth = mercenariesInfo[id].hp;
m.GetComponent<StatusC>().health = mercenariesInfo[id].hp;

bool full = player.GetComponent<SpawnPartnerC>().AddPartner(m);
if (full)
{
Destroy(player.GetComponent<SpawnPartnerC>().currentPartner[0]);
player.GetComponent<SpawnPartnerC>().Sort();
player.GetComponent<SpawnPartnerC>().AddPartner(m);
}
}
else
{
print("Please Assign Mercenary Prefab");
}
}
else
{
//When you have not enough cash to hire.
noCash = true;
}
}
}