Okay, I'm past my prime...
...I'm not to be messed with.
How about a plain old (fake) wedding ring? I think most people in India would recognize that as a sign of being married.
... as for persisitent annoyers - they won't be repelled by any such symbols.
As a big-city girl I developed the don't-mess-with-me walk and look early, and it's quite effective. My problem is that I love to talk to (actually, listen to) strangers.
Okay, I'm past my prime, but I am a woman traveling alone, and I'd like to temper my irrepressible gregariousness with some outward sign that I'm not to be messed with.
Do not try to "make friends" with the run-of-the-mill Indian man, don't smile at men on the street (in fact, don't smile on the street), and if you are having a conversation (like on the train), don't look men in the eye. Hang out with the women whenever possible.