I was looking for the same and I saw that nobody answered, so this is what I’ve found: you cannot and you won’t ever be able to.
@channel: everybody in a channel, it’s a kind of contract with the user and everybody inside a channel should receive a notification.
The point is that Slack has a netiquette, and users would have to know it. Alternatives to @channel keyword are @everyone and @here, the first one working only in “big” channels, the second one only for people currently active. You can disable notifications for these keywords.
https://get.slack.help/hc/en-us/articles/202009646-Make-an-announcement
