Good initiative, though poor in execution. You're asking to stop an action which has a high potential reward and effectively no risk. (There are no effective cultural, social, or legal consequences for asking, and few for possession).
That said, it does highlight the problem: all the pressure and all the risk is on the ladies' shoulders, not the gents. It's a double-sided trap with social pressure on one side and shame on the other.
What I would tell the boys?
Number one, know when to stop asking. If it's hard to determine, the first rejection and/or response that's anything other than enthusiastic participation is the time to stop asking.
I realize this seems counter-productive. But rest assured, you're not denying gratification that way, just delaying it. Shit, being able to take rejection graciously will do worlds more good for your rep than not knowing when to back off will.
Hell, half of the risque images I get these days I don't ask for. Just got some friends who have their own ways of cheering me up. And they know I won't give them shit about it when they don't, either.
Oh, and don't send a dick pic you haven't been asked for. I have no idea how "do unto others" got perverted into this shit, but seriously, stop.
What I would tell the girls?
The second he starts asking, start screenshotting.
Not my place to say whether any given knucklehead is worthy of turning the lens on yourself. But I will say that, sooner or later, these things look like they get ugly.
The second he starts getting entitled, or threatening?
Fucking blast those shots on every public forum you share. Every last one. Doesn't matter if he's already got shots of you. What people see first hits hardest, and taking the initiative is what lets people see him threatening you long before or even if they don't get a single look at you.
Yeah, it'll hit him, maybe hard. So what? Fuck him. The second he stopped taking no for an answer was the second he proved all he gave a shit about was your ass and his ego.
Remember the trap I mentioned earlier? Don't stand around and wait for engineers to dismantle it. You get sucked into the social pressure entrance, you blow the shame door right the fuck up before he ever gets a chance to close it on you.
Yeah, you'll probably get some blowback on it. Fuck it. It'll pass. In a year, he'll be the asshole who couldn't take no for an answer, and you'll be the girl nobody wants to fuck with.
They're not ideal answers, and they're off the cuff. But they're what I got at the moment.