The current workaround involves handling RadGridView's ColumnReordered event:

        private void RadGridView_ColumnReordered(object sender, GridViewColumnEventArgs e)
        {
            if (!e.Column.IsResizable)
            {
                Dispatcher.BeginInvoke((Action) (() =>
                {
                    var radGridView = e.Column.Parent;
                    foreach (var header in radGridView.ChildrenOfType<GridViewHeaderCell>())
                    {
                        if (header.Column.DisplayIndex == (e.Column.DisplayIndex + 1))
                        {
                            var leftGripper = header.ChildrenOfType<Thumb>().Where(x => x.Name == "PART_LeftHeaderGripper").First();
                            {
                                leftGripper.Visibility = Visibility.Collapsed;
                            }
                        }
                    }
                }), DispatcherPriority.Render);
            }
        }