To delete products imported from the BrandsSync app, you must first delete the products from the Shopify Dashboard.
Go to the 'Products' section and select all the products. From the 'Actions' menu select 'Delete selected products'. Repeat for the different pages.
After deleting all the products from the Shopify Dashboard side menu, select App > BrandsSync. Once inside the BrandsSync app window, select 'Settings' from the side menu and then select 'Force Full Sync'.
The duration of this operation depends on how many products were loaded on the site. When the operation is finished in the 'Home' section of the BrandsSync app there will be no imported products.
To follow the different steps watch this video