Currently it's not possible to specifically turn off the "@all" mention for a room or a user.
I found an idea in the HipChat Forum suggesting this to be implemented. Please consider voting for it and following the topic directly in it: https://help.hipchat.com/forums/138883-suggestions-ideas/suggestions/4443200-ability-to-administrativly-disable-all-for-a-room.
If this notification is becoming a problem for now I'd suggest reinforcing the users to avoid using the "@all" mention and, instead, use the "@here" mention, which would call the attention only of online users, without sending them an email. You can find further information on how mentions work at https://help.hipchat.com/knowledgebase/articles/64429-how-do-mentions-work.
Consider using HC Alias addon, to give room visitors alternative for @all. E.g. we have rooms per team, but other visitors like members of other teams can be in the room of another team, and they would not like to be notified through @all but still you want to allow visitors to address team members only. So using HC Alias addon in every room we defined @team alias with only team members, so visitors can address team only.
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