Simply hover over the "Library" tab at the top of the Steam application and click the "Home" tab in the menu.

Navigate to Call of Duty®: Modern Warfare® III in your steam library and right click on it. (the search feature can make this easier)

Hover over the "Manage" option and click on "Uninstall" (it's the last option)

When all steps go touch some grass or do something(What ever you want. You are free now). But what ever you do NOT install Call of Duty®: Modern Warfare® III back if you must play COD games play this:

Its so much better.

This will be a first day in your life without Call of Duty®: Modern Warfare® III and your amazing journey can begun! Enjoy!