Table permanently Sold out

I have the following problem. A guest reserves a table, so click on a place, it puts itself in the shopping cart, and closes the window again. Now the place is still displayed as reserved, although the guest has not completely completed its reservation process. Of course that's very bad, so fun birds can abuse the system. Is it possible that the place is displayed only as reserved if it completely completed the process? Otherwise, the whole thing has no sense for me. :-( Then the following I use this script for a bar, it would be possible 1. If a guest does not perceive the appointment that it is automatically deleted after 20 minutes? 2. That the guest can enter his own time when he would like to reserve and goes about? And as soon as he goes or earlier, the appointment will automatically be adapted for other guests. Thank you first.