The trouble here is that you can't pause execution in javascript, so you'll need to adjust your confirm function (which you should probably rename, by the way, since JS has a native confirm function). Get rid of the onclick of your yes button, and adjust your confirm function like so:

function confirm(message,nobutton,yesbutton,yesfunction){
  $('#overlaymessage').html(message);
  $('#nobutton').html(nobutton);
  $('#yesbutton').html(yesbutton);
  $('#overlay').show();
  $('#yesbutton').off("click").click(yesfunction);
}

Then, pass a function into your confirm call:

function deleteuser(userid){
  function deleteConfirmed() {
    // delete code here
  }
  confirm('Delete user?','No','Yes',deleteConfirmed);
}