The contract (you can think of it as a class in Java) has a list of private keys who are allowed to make modifications to the database. Each key has has a list of associated privileges - what modifications can that user make. Keys with a higher privilege level have the right to add/remove keys.

When someone creates a booking, he has the right to remove his own booking. Each booking might also have an expiration date, so it's removed automatically when expired. When a key is stolen, a key with a higher privilege level can disable access of the stolen key.

If it's implemented correctly (which will take time and maybe several failed attempts), hacking, bugs and DDOS should not be an issue.