Go to the price list of your event (see how to find it here).

Click on "in detail" in the "discounts" column (for the price zone you want), so that you can see the discounts table of the selected zone at the bottom.

For the discount you want to make the visibility change, click on the pencil icon in the column "active on teller - sites".

Select from the dropdown menu "not on website (only on teller)" if you do not want it to appear online or "on teller and websites" if you want it to be visible on your pre-sale page.

Hit ''OK''.