You can get into the invoice preferences to turn off the discount column. Following are the steps to do os;

• Go to Settings tab.
• Click the General Preferences sub-link under Preferences.
• Click No for the option "Do you offer discounts to your customers?" (under Discount).
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• Click the Save button.

If you would like to remove the discount column only for certain customers, then you can customize the invoice template and can delete the discount columns. This allows you to select the template for the customers while creating invoices. Steps as follows:

• Click the Settings tab.
• Click Invoice Templates sub-link under Preferences.
• Click the + icon in the blank template.
• Choose a template type for invoicing your customer and click on 'Use This'.
• Enter a Name for the template.
• Uncheck the discount field shown under the Item Grid Labels.(Refer the sample attached screen-shot).
• Save the template and select it as your default template.