Eli A., are you on a free workspace or a paid Slack workspace?
In a free Slack workspace, you only get one channel that allows you to control posting permissions. This is usually called the #general or #all-xxxx channel. For us it's Announcements.
Let me know if you have any more questions
Mostly through strong community guidelines and best practices. I would say a majority of our channels though welcome participants.
The only one that doesn't is Changelog but we find most folks respect not posting in there