Instructions
Go to "My Xbox" and scroll over to "System Settings."
Choose "Memory" and then pick the device on which your files are stored -- hard drive or memory unit.
Select the "Games" category and scroll down to the correct "Call of Duty" file.
Open the game file to view the subcategories of files contained within it. These files should mostly be game saves and DLC. Look for files described as "Marketplace Content." These are mostly downloadable content.
Pick a file and then choose the "Delete" option to remove it from your memory device. Repeat this process if you have multiple files to delete.