Thanks Ipstenu, that worked like a charm. The "Gimme a site" option is indeed checked by default, so I just hid it as you described.
I wasn't looking for support here, for wpmudev's plugin, but just making reference to my having used it some time ago. I've been jumping between this and their forums trying to work this out. Actually, they do "bloody well" provide excellent support. In fact, they pointed out that the plugin is not theirs, but rather a free plugin that can be found right here.
So between hiding the selections as you describe, and using the above plugin, it's pretty easy to force sign-ups for single blogs only.
Once I had that set up, I did a test sign-up, only to discover the second step in the sign-up process includes choosing a URL for their newly created blog. But the text field is already filled with the username they created, so to prevent them from changing it I simply hid that option by adding class="hidden" and let the CSS take care of it.
Thanks again for all your help. :)