Speaking as a non-bride, I can't help but think of this from a guest's perspective. Imagine if this was you. You get a "Save the Date!" in the mail, and think, "Yeah! I deserve a vacation, and I'll get to see my friend/ family member get married too! How nice!"
And when you try to book, you are refused... because the bride "didn't think you would really come"?? I would be so hurt and feel like a second-class person. There is little sense in inviting people if you don't really care if they come or not, and start to fret about how much they are costing you if they dare to take you up on the invitation you made.
I honestly think you have to live with however many people do decide to come, and that there is no way to "politely un-invite" people. Sorry.