By default, in Windows 7, recently opened programs, files and other items are listed in taskbar when specific applications or folder are right clicked.
Example : Right clicking on an Excel sheet in taskbar lists all the recently opened spreadsheets as shown.
To prevent this from happening :
1. Right click on taskbar and select Properties.
2. Go to Start Menu tab and uncheck the option “Store and display recently opened items in Start menu and the taskbar.
This will now prevent listing recently opened files, folders etc when right clicking a specific item or an application in the taskbar as shown below.
To prevent the recently accessed items from showing up in Start menu, check this earlier post.