In our experience, this is just a scare tactic from the Yext sales team. The primary “lock” they’re interested in is locking you into your ongoing subscription with Yext.
You do not need to worry about locking your listings. Once they’ve been cleaned up properly at the primary sources that feed other sites, they’re not going to be overridden with incorrect information. This is why we focus our cleanup on the core 11 sites in our Yext Replacement Service. When your data is accurate on the primary sources, it will remain accurate across all the sites that pull from the primary sources.