How do I disable auto join in Slack? I keep getting invited into a channel that I don't want to be a part of and it's getting really annoying.
I believe this is a misunderstanding. There is no auto-join feature in Slack (expect for people joining a Slack for the first time). So whenever you get invited into a channel its because a user or an app has explicitly invited you. It will say in the channel who invited you, so you know whom to contact.
Many companies have apps that make sure people stay in important channels (we have the same on our Slack). In that case you need to talk to the admin who runs the app.
You can look into setting up a script that leaves a channel using the api: https://api.slack.com/methods/channels.leave