What if I told you that not wanting such attention, and not wanting to give such attention back, actually makes most men lose interest in you? And that it's actually a test.
Does this bother you? That what most women seem to find offensive, is seen as attractive by the opposite sex?
What if I told you that any guy who spams or insults you when you don't reply (or when you say not interested), actually isn't interested in you at all, and simply gets off on trolling anyone (those guys are rare though).
To be clear, I've never hit on a woman in my entire life, online or off, and I don't trust anyone (or the net) enough to send pics, but I have been hit on by women, I've gotten lewd messages and pics I didn't ask for, and I found it to be an endearing act/personality trait (even if I wasn't interested in her in the slightest), and most guys I've met feel the same about it.
Which I suppose is why they message with unsolicited flirting.