I've been doing cam sex, sending dirty photos, and talking dirty with a guy for a while. But I don't really want to continue anymore. I want to tell him, but I fear that he will spread what I sent or blackmail me to make me stay and continue. What do, guise?
Fake your own death/tell him you no longer want that sort of relationship. Blackmail is a crime, screenshot any conversations that sound blackmail'y and contact the police if it comes to that.
Does he know your circle of friends or do you know him irl well otherwise? If not just tell him. If anything gets posted online, if it's not your facebook I doubt it's gonna affect you in any way
Does he know your phone number or any methods of contacting you offline? If so that might make things difficult.
Something you can do if he doesn't is just disappear. Drop that online handle, move away from it, block him, etc. If you can continue to monitor his messages while appearing offline, that can give you a good idea of what his reaction will be to you just jumping ship. Things like overly aggressive messages, threats, etc. Then you can A) reinsert yourself into conversation with a "sorry my internet was out!" and B) start working with authorities or other bodies to gtfo of there.