Ah. Well you could try another one. Though that's time-consuming. Or uhm, maybe try something like uhm... get them to watch a good explanative video? One not made by a trans person. Because, well, you know, I sort of understand them. When you don't want to believe something, someone who in any way seems "sold" to you will have much less credibility to begin with. In this precise case they're displaying bad will, but if you look at it that way, it sort of makes sense.
Just going forward with your things however you can and acting mature and "above" them should, normally, in time, get them to a more open state where they'll try to understand what's happening rather than stop it. Basically, if they stop feeling like it's within their power to stop it, they should stop trying to stop it. By acting above them, basically, I mean having confidence in your decision and affirming how ridiculous what they're saying is if it is, but in a mature way. That part was easier for me because my mother's denial stage was pretty minor and for whatever is scientific, technical or theoretical she generally accepts that I'm better than her in the domain to begin with, but I think that it should be possible without those "bonuses".
All I'm saying is pretty vague and doesn't give you any guarantees. I wish I could help more.