Using the lasso select tool you can select an area on the map and un-assign all the accounts that are located in the lassoed area.
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 only accounts make sure to have "only Accounts" selected as seen in the image above
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 Austin.
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 accounts you have selected will be set to unassigned. To confirm type F5 on your keyboard to toggle the Account View. The accounts that are displayed in Black are the accounts that are un-assigned.