Just like the "Recent Tests" list, the "Recent Testbanks" list is self-generating and shows the last five items you opened. As you open new testbanks, the most recent testbank will be added to the top of the list, bumping down all the previous items.

If you really want to clear the list, though, one thing you could do is choose "Reset TestGen" on the Tools menu. Caution: This will set all of the program defaults back to the way they were when you installed the program, including turning testbank editing off. When you restart the program, you will need to turn testbank editing back on from the Application Preferences dialog. Other preferences and defaults that you have set may also need to be reset.