You can 100% mute and block people on Twitter, Facebook, Instagram, most main line social media platforms. Hell you can even mute keywords on Twitter and never have to deal with seeing another tweet about Will Smith slapping someone.

Should one user be able to decide if another user should be expelled from a platform? That leads to a lot of sticky problems, so no, they shouldn't have that power. Should a user be banned if they violate the terms of the platform (roughly what mods on IRC would do when a user was banned)?

Yes. Yes they should. And they do. It's a liability for platforms to not enforce their own rules.