Where in the bible does it say not to show your nakedness to someone who is not your spouse? (I can't find it)
There is this guy who keeps asking me to send him pictures of myself. I keep telling him no, but he won't leave me alone about it. I want to send him the verse about it so he will hopefully stop.
Whatever medium you are using to communicate with this person there are things you can do to simply end it, if it's a phone then get a new number and guard the new one carefully, if it's email you can block his address, if you don't know how make the effort to find out.
If he keeps coming back it seems clear he won't be reasoned with, he probably knows it's wrong but wants the pictures and so has made his peace with the fact that it's wrong and has decided to embrace his desires anyway, so don't waste anymore time trying to convince him, you likely won't succeed.
If you say no but continue to communicate he will, in his own way, take that as encouragement to continue asking, because from his viewpoint it will seem like you are saying no all the time but if you were really that against it you wouldn't keep responding to him.