When an ad network refuses to pay the publisher for displaying their ads without a reason, the publisher is forced to remove the ad code from the website, so that other networks who pay are able to show their ads better.
The publisher is paying a large amount of money for domain renewal, webhosting and other expenses, so if the ad network does not pay the ad code is removed .
If the ad code is used on a large number of websites, removing the code may take more time, but the code will get removed within a few months
Unitraffic.ru was the latest ad network, to stop paying publishers and now the code has to be removed.