There are no registry keys that can be implemented to make the PST Locator scan more than one computer at a time.
The Log On account used for the PST Locator Task must be a Domain Administrator or a part of the computer's Local Administrator's group. Verify that the Log On account for the Locator Task is not the Vault Service Account.
Verify that file and print sharing is enabled and there is an admin share available for scanning (C$, D$, etc). Even if the scan registry is not selected the Locator Task still has to scan the registry for the drives that it needs to scan.
Verify that there are no group policies applied to these computers that prevent a registry scan.
If you do not want these computers to be scanned then change the Computer settings in the Vault Admin Console and remove the "Search this computer when the PST Locator task runs". The Locator Task has a setting to automatically search for PSTs on each computer. You may want to disable this setting and individually search specific computers.