simonmitas,
Removing the viewport meta tag like you did is the first step, but you also have to remove the responsive styles in your CSS stylesheet. Look for anything that begins with @media and delete that from your stylesheet. This should do the trick.
I suggest you backup your stylesheet completely in case you decide to make your theme responsive again. Also keep in mind that responsive themes are more user friendly and better for mobile users, so consider that before you remove all responsive styles.
Hope that helps!
