"Also the paradox that you might need to keep details of a customer, marked with a flag to say that they don't want to be contacted by marketing."
Exactly the problem I'd been thinking of! Somewhat reminiscent of this...
LISTER: Holly, is there something that you want?
HOLLY: Well, only if you're not busy. Would you mind erasing some of my memory banks?
LISTER: What for?
HOLLY: Well, if you erase all the Agatha Christie novels from my memory bank, I can read 'em again tonight.
LISTER: How do I do it?
HOLLY: Just type, "HolMem. Password override. The novels Christie, Agatha." Then press erase.
LISTER jabs two-fingered on a keyboard.
LISTER: I've done it.
HOLLY: Done what?
LISTER: Erased Agatha Christie.
HOLLY: Who's she, then?
LISTER: Holly, you just asked me to erase all Agatha Christie novels from your memory.
HOLLY: Why should I do that? I've never heard of her.
LISTER: You've never heard of her because I've just erased her from your smegging memory.
HOLLY: What'd you do that for?
LISTER: You asked me to!
HOLLY: When?
LISTER: Just now!
HOLLY: I don't remember this.
LISTER: Oh, I'm going to bed. This is gonna go on all night.