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

For example, if you wish to see the status of the invoice emails, you could click the "Show" drop-down filter and can select invoice notification from the list.

• 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).
• 
• 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 New Template button.
• Enter a Name and Description for the custom template.
• Select the column and click the delete icon (Refer the sample attached screen-shot).
• Save the template and select it as your default template.