To make sure that the information isn't stored in the Protect database you can follow the steps that were previously given in this thread: http://community.shavlik.com/thread/447608?tstart=0
Otherwise, the information would be getting pulled from somewhere on the network during discovery of machines in the scan process.
One thing you could do is once you run the network/domain scan - add the systems you know you will want into a new machine group, then just delete the information of the unwanted systems. (You can add systems to a new group by viewing in Machine View, highlight systems, right click, and choose 'Add to Machine Group'.
How can I delete the information of the unwanted systems? Ony machines that were successfully scanned are in the machine view so I cannot see any of them there.
The information of systems that were not scanned would only be in scan results. If you delete the scan results (from manage > items or database maintenance) it will delete the associated data.