@Nahum1 AirBnB always asks the host before adding a special discount (such as the 18% discount for first guests).
Once you approve a discount, you usually can remove it by changing the calendar (as you have tried), but I think the 18% discount for first guests is not tied to any specific calendar dates. This is probably why the discount is still active, even though you tried to remove it from the calendar.
I do not know if you can remove this discount yourself. It may not be possible.
But you can ask AirBnB support if the support people can remove it for you.
(But be aware they may not be able to do it either.)