You would need to customize the products_quantity_discounts.php module, specifically the section around line 132:
$quantityDiscounts[$columnCount]['show_qty'] = number_format($products_discounts_query->fields['discount_qty']);
$products_discounts_query->MoveNext();
if ($products_discounts_query->EOF) {
$quantityDiscounts[$columnCount]['show_qty'] .= '+';
} else {
if (($products_discounts_query->fields['discount_qty']-1) != $show_qty) {
if ($quantityDiscounts[$columnCount]['show_qty'] < $products_discounts_query->fields['discount_qty']-1) {
$quantityDiscounts[$columnCount]['show_qty'] .= '-' . number_format($products_discounts_query->fields['discount_qty']-1);
}
}
}