<div class="messages-box">
{{#each messages}}

    {{#if isCurrentUserCan}}
        <p>{{msg that user can do bla bla}}</p>
    {{else}}
        <p>{{msg that user can't do}}</p>
    {{/if}}

    </div>
{{/each}}
</div>

Template.body.helpers({
'isCurrentUserCan': function(){
  if ( random message box's user ID == Meteor.userId() ){
      return 'This user can do bla bla';
 }else{
      return 'This user can't do';
 }
}
});