In the space, go to Space Information , and then click Disable @mentions to all.
You can only turn it off on desktop, but it's turned off for everyone using Webex on mobile too.
Webex | Turn Off @all Mentions
Sometimes you may want to control the amount of @all messages that are sent in a space. If you're the moderator, you can disable @all, to prevent people from mentioning everyone in the space at the same time.