Generally, inFlow doesn’t let you delete things entirely from the database.  This way, you have a full history of everything.  For products, customers, and vendors, you can open it and click the Deactivate button in the toolbar.  This will hide it away so that you won’t normally see it again later.  If you want to use the same Item Name/Code or Customer/Vendor name later on, you can rename it something else first.

For Sales Orders and Purchase Orders, you can Cancel the order.