Sorry to hear your parents are being like that and I totally understand how you feel.
My parents disowned me at 24 when I went for SRS and they never relented. I was not allowed back home, except for two short visits over 35 years. The most stupid part was that everybody in town knew from the time I was young so it was no secret. It was simply out of spite, to try to hurt me, but it was THEIR loss! The lost the chance to know someone they could have been very proud of.
Hang in there kid, they may yet relent, particularly if they think you are going to stick to your guns and not back down.