How to mark a sector as bad?

I want to make a protection floppy disk, that means i have to recognize it, and to ensure that it is not faked. That why I want to mark a certain sector as bad and while the protection is running it will check if the certain setor is marked as bad.
if u know how to mark a sector as bad, how to detect weather a certain sector is a bad one or if u have any other smart way to recognize a floppy disk, please help me.

Form Niztan Aviram.
Any help whould be highly appreciated.