By deciding what is and isn't allowed. If people ship Bakugo and Deku together who are you to attack them for it? Instead, ask why they thing it'd be a nice think to see, and then present a counter argument.
It's like the RWBY fandom, many people think that shipping Blake and Yang together is cringe, yet people still do it. Why? Because they are protected by freedom of speech. I dislike the Bumbleby ship, yet I don't go around attacking people for it.