When I decided to stop going by the diminutive form of my name, I just stopped answering to it for several years. It mostly worked, though I don't know if I'd have been ballsy enough to cause that much friction if I'd been older. My dad always very pointedly used my full name after that. When my sister's kids were little and used the diminutive it was clear the full name was something the family only used when speaking to me, and not when speaking about me, but I was okay with that, honestly. They're the only people I respond to when they use it; everyone else, I honestly think they're talking to someone else.