Using the lasso tool you can select an area on your map and then choose to un-assign the zip codes and accounts all at the same time.
Select the Lasso select tool from the home menu. You can also use the Circle select tool, but for this example we will only use the lasso select tool
In order to select and un-assign zip codes and accounts make sure to have both of the selected as seen in the image above.
Now choose an area on the map that you would like to un-assign. Using your mouse left click on the map and now lasso the area and let go. In this example I will select an area in Nashville.
As soon as you let go a pop will appear with a summary of the newly lassoed area. Now click Next.
Now, simply select the third option (1) Unassign Zip Codes and Accounts and then click Next (2)
As soon as you click next the zip codes and accounts you have selected will be set to unassigned.