You cannot delete a product or item kit if you have placed orders with it, even if those orders have been fully fulfilled.
You will need to disable this product/ item kit.
How to delete:
Step 1) Go to the "General" tab for the product / item kit
Step 2) On the right hand side you will see a blue button with the words "delete"
How to disable:
Step 1) Got to the "General" tab for the product/ item kit
Step 2) Under the blue line with the words "General" you will see the word "Disabled" and the word "No" under it.
Step 3) Click on the word "No" and change it to "Yes"
