When you pre approve your bookings, then the guest can book. If you block those dates, then they wouldn't be able to.
What you can not do is pre approve someone and then block other potential guests from inquiring about the same dates. You could decline them if you want to save the dates for the original person, but I would not recommend it. Sometimes someone will inquire, then I will pre-approve, then they don't book anyway.
I learned this the hard way. I felt sorry (and a bit responsible to provide the room) for the person who requested me first, then they didn't book. Now I just pre-approve, and whoever books first gets the room. You can let them know that someone is inquiring about the same dates (which they may or may not believe..)