It's not crazy to ask a boyfriend to stop talking to his ex-girlfriend if its making you uncomfortable. Even if that ex-girlfriend is still in his life, it's not crazy to ask. Don't make a big deal out of it, and don't be angry about it. Just state what you want - "You've been talking to your ex a lot, and I was hoping you wouldn't as much. I know that she's still your friend, but it hurts me to keep hearing her name again and again, and to know that you're always talking to her."
There, that's it. Your boyfriend will know immediately how it makes you feel, and hopefully he'll change how he talks about her, and how much he talks to her.
Since, however, that ex is still in his life, he's still going to hang out with her from time to time, and you need to be alright with that. Hopefully, he won't be spending one-on-one time with her, but they will be in the same place at the same time. If you're not alright with that, then perhaps you need to change your mindset or your boyfriend.