Several people have found a work around for this question by creating a discount called 'none' with a % of 0, but this isn't the best approach because your estimates will still show a discount field, simply with the word 'none' and that's not what you want your customer to see.  Instead, click directly into the discount field and then backspace and delete the discount word entirely.  When the discount title is gone, then tab away, and the discount amount will also disappear!