Don't register Mouse Clicks through GUI.

Hello, I have this code. The code is creating a window and pausing the game, the window contains the skills I want to increment. The menu is working great, I only have a problem with it.

When I want to increment the skills the mouse clicks registers through the GUI into the world and I want to prevent that, I only want the clicks to register in the GUI when the GUI is displayed.

Can someone help ? :)

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 
 public class LevelUpSystem : MonoBehaviour {
 
 // Values For Attributes
     [HideInInspector]
     public int attack, defence, manaPool, manaRegen;
 
     private int lvl;
     private const int STARTING_POINTS = 3;
     [HideInInspector]
     public int currXP = 0;
     [HideInInspector]
     public int maxXP = 40;
     private int pointsLeft;
     public GUISkin mySkin;
     public int xpIncrement = 20;
     public Texture2D levelUpTexture;
     public Texture2D cancelButton;
     public Texture2D acceptButton;
 
     public GameObject levelUpEffect;
 
     private List<Skills> skills;
     /**********************************
      * Character Window Variables
      * ******************************/
     private bool displayCharacterWindow = false;
     private int charWindowID = 2;
     private Rect characterWindowRect = new Rect (Screen.width/3,Screen.height/3, 512, 512 );
     private int characterPanel = 0;
     private string[] characterPanelNames = new string[]{"Attributes","Skills"};
     private ProjectileDirectDamage damageRec;
     private PlayerHPMPSystem manager;
     private bool levelUp;
 
     void Start()
     {
         levelUp = false;
         manager = GameObject.FindGameObjectWithTag("Player").GetComponent<PlayerHPMPSystem>();
 
         skills = new List<Skills>();
         skills.Add(new Skills("Attack", attack, 10));
         skills.Add(new Skills("Defence", defence, 1));
         skills.Add(new Skills("Mana Pool", manaPool, 10));
         skills.Add(new Skills("Mana Regen", manaRegen, 1));
         
 
 
         pointsLeft = STARTING_POINTS;
 
     }
 
     void Update ()
     {
 
         if (currXP >= maxXP)
         {
             LeveledUpSystem();
 
         }
     
         if(Input.GetButtonDown("CharacterWindow"))
         {
 
             ToggleAttributes();
             
         }
 
         
 
     }
 
     void LeveledUpSystem()
     {
         levelUp = true;
         GameObject myLevelUpEffect = Instantiate(levelUpEffect,transform.position,transform.rotation) as GameObject;
         myLevelUpEffect.transform.parent = transform;
         currXP = 0;
         maxXP = maxXP+xpIncrement;
         lvl++;
 
 
 
         pointsLeft += 3;
         manager.maxHealth = manager.maxHealth + 20;
         manager._currHealth = manager.maxHealth;
 
     }
 
 
     void OnGUI()
     {
         #region Character Window Display
         GUI.skin = mySkin;
 
         if (displayCharacterWindow)
         {
 
 
             characterWindowRect = GUI.Window(charWindowID, characterWindowRect, CharacterWindow, "Character"  );
 
         }
 
 
 
         #endregion
         GUI.Box(new Rect(5,100,40,20), "XP");
         GUI.Box(new Rect(5,120,40,20), "Level");
         
         GUI.Box(new Rect(45,100,100,20), currXP+"/"+maxXP);
         GUI.Box(new Rect(45,120,100,20), lvl+" ");
 
         if (levelUp == true)
         {
                 
             GUI.Label(new Rect (50,350,100,25), "Level Up");
             if (GUI.Button(new Rect (50,300,50,50), levelUpTexture))
             {
                 ToggleAttributes();
             }
             
         }
         if (pointsLeft <= 0)
         {
             levelUp = false;
         }
         
     }
 
     public void CharacterWindow(int id)
     {
 
 
         characterPanel = GUI.Toolbar(new Rect(5,20,characterWindowRect.width - 10, 30), characterPanel,characterPanelNames);
 
         switch(characterPanel)
         {
         case 0:
             DisplayAttributes();
             break;
         case 1:
             DisplaySkills();
             break;
         }
 
         GUI.DragWindow();
     }
 
         private void ToggleAttributes()
         {
             displayCharacterWindow = !displayCharacterWindow;
 
             if (displayCharacterWindow)
             {
             Time.timeScale = 0f;
             }
         else if (!displayCharacterWindow)
         {
             Time.timeScale = 1f;
         }
             
         }
 
     private void DisplayAttributes()
     {
         GUI.BeginGroup(new Rect(5,70, characterWindowRect.width-10, characterWindowRect.height-70));
 
         GUI.Label(new Rect(10,0,100,25), "Points Left"+pointsLeft);
 
         for (int i = 0; i < skills.Count; i++)
         {
             GUI.Label(new Rect(0,40 + (i*25),100,25), skills[i].name );
             GUI.Label(new Rect(80,40 + (i*25),100,25), skills[i].value.ToString() );
         //Buttons
 
             if(GUI.Button(new Rect(100,40 + (i* 25),25,25), "+"))
             {
                 if (pointsLeft > 0)
                 {
         
 
                     skills[i].LevelUp();
 
                     pointsLeft--;
                 }
             }
 
 
         }
 
         if(GUI.Button(new Rect (characterWindowRect.width/2,characterWindowRect.height/2, 198,63), cancelButton))
         {
             ToggleAttributes();
         }
 
         if(GUI.Button(new Rect (characterWindowRect.width/10,characterWindowRect.height/2, 198,63), acceptButton))
         {
             ToggleAttributes();
         }
         
         GUI.EndGroup();
 
     }
 
     private void DisplaySkills()
     {
         Debug.Log("Displaying Skills");
     }
 }