First off, you need to get a reference to the GUITexture's game objects. You can do this in one of several ways:

• Find the object by name
• Find the object by tag
• Store a reference to the object in your camera script using the Inspector, and a public variable.

Assuming you have the reference to the game object(s) containing the GUITexture, turning it on and off is relatively easy.

// C#

// GameObject myObject;
// Get a reference to the game object that stores the GUITexture somehow.

GUITexture gt = myObject.GetComponent(typeof(GUITexture)) as GUITexture;
gt.enabled = false;

Once you have the component reference, just set enabled to false, and it'll stop whatever it's doing immediately.