Default How do you block OTA updates with root?

Besides installing a custom rom of course.. I just rooted my girlfriend's phone after she saw a notification today for an OTA. Saw some of the verizon guys suggested freezing SDM.apk with titanium backup, but I couldn't find it in the list. Others mentioned just renaming it with the .bak extension, but once again I couldn't find it in the /system/app partition using ES file explorer. Am I missing something? Is there a better way to prevent losing root via OTAs? Thanks in advance!