You can ONLY void created vendor returns. You CANNOT void completed vendor returns.

Navigate to the Dashboard.

Select the Vendor Return tile from the "Parts and Accessories" tile set.

Select the Vendor from the Vendor drop down menu.

Select Created from the Status drop down menu to display all created orders (meaning orders without an RMA#).

You may want to click the listing to verify the correct return is being voided. 

Select the ... icon.

Click Void.

Click Ok to confirm.                            

If you are certain that the listed entry is the return you want to void, click the Trash icon from the list. This item is now voided.