Go to your Personal Account.

On the website: go to the Settings tab.
In the app: find the Receive notifications about promotions and discounts block at the bottom of the screen.

On the website: uncheck the box next to I agree to receive newsletters from Kupibilet.
In the app: uncheck the boxes for the notification methods you do not wish to receive.

On the website: after changing the data, do not forget to click the Save button.