@Aesica : That was exactly the problem Starflight had with their save method. They saved everything to the main disc, and if you got saved into an unwinnable situation the only way to fix that was to...buy the game again. Or have made a backup of your discs.
Needless to say it did not go over well, even with the developers posting a warning to make a backup of the discs and only play on the backups.
As for the question asked, you can actually do what you want with restricting the game to one save slot only (and I think there are many plug-ins that let you decide how many save slots you want, I just don't have links to them as I mainly use ACE). No need to force them to delete it if they hit restart, as if they do hit restart they either go to ironman the game or overrite the save once they decide to save.