Unfortunately, it seems that if it had any effect it was short-lived. Looking at anti-spam blocklists on a daily basis, a couple of Verizon Autonomous Systems (ASes), AS-19262 and AS-701, do show dips in blocklist listings on the blocklist PSBL in March. But they don’t last.
Spammers are very adaptable, partly because the botnets they use are adaptable. Good try, Verizon.
This information is from an NSF-funded academic research project at the University of Texas at Austin business school. Thanks to PSBL for the blocklist data.