There are 2 ways you could go about doing this:

1) Use InitializeRow:

private void ultraGrid1_InitializeRow(object sender, InitializeRowEventArgs e)
{
    if (e.Row.Index >= 100)
        e.Row.Hidden = true;
}

2) Use a custom FilterCondition:

private class First100RowsCondition : FilterCondition
{
    public override bool MeetsCriteria(UltraGridRow row)
    {
        return row.Index < 100;
    }
}

private void ultraGrid1_InitializeLayout(object sender, Infragistics.Win.UltraWinGrid.InitializeLayoutEventArgs e)
{
    e.Layout.Bands[0].ColumnFilters[0].FilterConditions.Add(new First100RowsCondition());
}

-Matt