My Deam Vacation

There are 3 of us travelling on an airline ticket that I paid for with Spiritair. One person might not be able to make it and the cancellation fees would cost as much as the airline ticket so we aren’t going to bother cancelling the ticket (she just won’t show up – although we could still check her in online 24 hrs before). How do we keep that seat empty (for extra room since we paid for it)?? If she doesn’t show up, can they still give it to someone else and will they cancel the return portion of her ticket automatically as she is a “no show” (but technically I should be able to keep that seat empty there and back because I paid for it). How do I do that?
I just called the airline and told them that I PAID for the seats and if a person doesn’t show up, I want the extra seat kept empty for more room. They told me that I paid for it and even if they don’t show up, let them know at the airport you want the extra seat empty because you paid for it and it won’t be a problem. Is this incorrect information?? Can they release the seat even if I have paid for it (didn’t cancel the ticket and spent the money on it) and want it kept empty if the passenger whose name is on it doesn’t show up?
What if I want the seat empty on the way there and back? If I didn’t cancel and made it clear that I paid for it and wanted the extra room, can they give it away anyways? The airline told me on the phone that I would be able to keep it empty because I paid for it if I told them at the airport….is this incorrect? Am I going to have problems doing this?
Thanks for the responses. No, transferring the name on the ticket is not an option – I was just hoping they would allow me to keep it in the customers name but leave the seat empty since I paid for it. Thanks again!!