Can I tell people Im infertile to get them to shut up about having kids?

No don't tell them that because they'll just pester you about "When are you gonna see the doctor about your fertility problem?" Telling them you're infertile implies that you checked, meaning you were unsuccessfully trying to have kids. Better to just let them blab, you've already told them you don't want kids and they aren't listening so it's just one of those things you'll have to let roll off your back.