To prevent or avoid users selecting “All with access” in Salesforce Chatter Post, we can develop a Apex Trigger on FeedItem object/entity and throw an error.

Please check the following sample Apex Trigger for reference.

Sample Apex Trigger:

trigger FeedItemTrigger on FeedItem ( before insert ) {
    
    for ( FeedItem objFI : trigger.new ) {
        
        if ( objFI.Visibility == 'AllUsers' ) {
            
            objFI.addError( 
                'Please don\'t select All with access' 
            );
            
        }
        
    }

}

Output: