Sorry, I'd keep the name and tell them to fuck off here like
It's not a corporation and I assume they hold no legal right to the name, I've also never heard of many groups or even companies where a founding member leaves and insists that the group/company changes its name.
It sounds (from my admittedly outsider point of view) that they're being completely unreasonable and I don't believe they should be given the kind of respect that would allow them to dictate the group now they've gone.