I know what you mean. I remember two occasions like this specifically - once I got trapped in my small office by a contractor telling her gory birth story (I couldn't have escaped without either diving out of the window or pushing her out of the doorway!) and another time, had to stop sitting in my own garden due to the loud, lengthy and gory story being related by not even my next door neighbour but the one the other side from that, to some not very impressed looking visitors.
I was also forced to volunteer to wash up at the 'preparation for childbirth' yoga class I did, as although the class was excellent, every week someone was invited in to tell her gory story at the end.
My birth story would probably sound gory if I told it but (this is completely true) I found it all very exciting. Looking back bits of it even sound funny, such as when my midwife got my wheelchair stuck, briefly, in some automatic doors as she took me to the delivery suite. The gas and air also made me feel completely stoned to the point where I was telling the surgeon I'd never wear a bikini again and she assured me quite seriously that she was renowned for the precision of her work and that I must promise to do so...
(DH didn't enjoy the experience at all but if it had been up to me I would have banned him anyway, as he is a softy).
Anyway, I think you need a range of strategies from 'oh my goodness, is that the time?' to 'haha, it's almost like you're trying to put me off?' or 'and you had a second?'
Finally, I mean no disrespect to people who feel a need to tell their birth story -- I think some of them maybe needed counselling that they did not get - they didn't feel listened to at the time so they're going to make you listen now, or something?