I know "@" is "at". But "at" can't be a verb in English. So, if someone in a chat group @ all members. You want to stop him / her. You can just say
Don't at all members in the group.
How do you say that properly?
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Sign up to join this communityBut "at" can't be a verb in English
Fortunately, English is flexible enough to allow words to take new or unsual usages. As you've noticed, it is in fact used as a verb in informal chat/social media lingo.
In your example it roughly means don't __________ all members in the group.
It depends on the context and platform you're using.
As a side note, there's the phrase "don't at me." I see how a learner could confuse it with the above. I found an explanation that also covers your original question:
The phrase is pronounced “don’t ‘at’ me.” The “at”-symbol (@) refers to the tagging of someone’s username, so that they get a notification of the engagement.
On Twitter, users cite this phrase after sharing an unpopular opinion or hot take. The idea here is that their statement will stir up much controversy and discussion, but they do not want to be a part of that dialogue. (Stayhipp)