I'm not going to let my boyfriend have a man bun at our university ball
What is wrong with a man bun?
I don't like them and they are gross.
That is your bf's style and you shouldn't diss it. You should be respectful of their taste.
The look makes me want to cut it off with a sword.
It is understandable to not like a style but you also need to respect his wishes.   You can talk to him nicely about it, but should be be aggressive or make it an ultimatum.