How to make him stop flirting?

My boyfriend still kinda flirts with his ex and it bothers me, he has also cheated on me with her.

And why are you still with him? He obviously isnt over her. Why would you want to be with someone who isnt into you 100%? Relationships has to have trust as a bases. If he cheated on you with her before...who says he wont do it again? You are basically allowing him to cheat if you stand by him. He figures it'l be ok to do, since your not going anywhere. "Dont make someone your priority, when your only their option".

well dup him there is someone better he might b useing you to make her jealous

break up with him obviously he doesnt have respect for you and I no I dont no you but I no you dont deserve that noone does

Well you never know what your boyfriend was going through but once he crossed the line he didn't take your feelings into consideration. I would fist warn him and really put your foot down. People make mistakes. Then I would see if he makes the change or not. If he doesn't I would leave because I don't tolerate that kind of treatment.