In order to remove Unstack from your Shopify account you need to remove the Unstack Sales channel. This is done by following the below steps which have been taken from this Shopify Guide.
- From your Shopify admin, click Settings, and then click Sales channels.
- Click Remove beside Unstack.
- Provide a reason for uninstalling.
- Click to confirm.
- The Unstack Sales channel should now be removed from your Shopify store.