You can delete a supplier for an existing opportunity by following these steps:
1. Click on the Contacts Menu.
2. Click on the Suppliers option.
3. Search for the supplier.
4. Click on the Supplier name.
5. Click on Tools/Actions button to view the delete option.
6. Click Delete this Supplier to delete the Supplier.
Note:
You will not be able to delete a Supplier if it is used by one or more Costbook Items.