Mary, thanks for your reply. I also saw, and appreciate your comment on MDL-19190.
First, no, my site is not open to Google. I am using the Default=No.
Likewise, "never modified" doesn't work because the spamming system registered them as users, and theirfore their user account (in Moodle's eyes) has been modified.
If you look at my attached file, those users not in grey are bogus. Look carefully at their city and country, and you will see how clever these spamming systems are. Even the names can appear real.
At this point, the only real difference between these bogus users and real users (as far as I can tell), is that these bogus users are not in any course. Being able to identify users not in any course should be a very useful feature in Moodle. Although I think this one feature will solve my own problem, it would also be helpful to be able to filter users by those within any specific course. My guess is that a more robust filtering method like this will take more time for someone to program because it should include "is equal to" and "isn't equal to" conditions, along with "is blank".