Your block settings are located under My iOffer > My Account > Preferences > Block Members/Countries.
Check the appropriate boxes to block negative or non-paying buyers from clicking "Buy Now" on your item listings. Or, enter the buyer's username to block them from purchasing your items completely.
Select the countries you want to block from purchasing your items by adding them to your block list.
Review your block lists and make sure you click on the Submit button to save your changes.