you can edit a core file or you can use a simple pluign. Its easier to edit the file. Open /inc/functions.php and find "fetch_ban_times".
Locate this:
// Days-Months-Years
$ban_times = array(
"1-0-0" => "1 {$lang->day}",
"2-0-0" => "2 {$lang->days}",
"3-0-0" => "3 {$lang->days}",
"4-0-0" => "4 {$lang->days}",
"5-0-0" => "5 {$lang->days}",
"6-0-0" => "6 {$lang->days}",
"7-0-0" => "1 {$lang->week}",
"14-0-0" => "2 {$lang->weeks}",
"21-0-0" => "3 {$lang->weeks}",
"0-1-0" => "1 {$lang->month}",
"0-2-0" => "2 {$lang->months}",
"0-3-0" => "3 {$lang->months}",
"0-4-0" => "4 {$lang->months}",
"0-5-0" => "5 {$lang->months}",
"0-6-0" => "6 {$lang->months}",
"0-0-1" => "1 {$lang->year}",
"0-0-2" => "2 {$lang->years}"
);
and comment out what you dont want, like this
// Days-Months-Years
$ban_times = array(
"1-0-0" => "1 {$lang->day}",
//"2-0-0" => "2 {$lang->days}",
//"3-0-0" => "3 {$lang->days}",
//"4-0-0" => "4 {$lang->days}",
//"5-0-0" => "5 {$lang->days}",
//"6-0-0" => "6 {$lang->days}",
//"7-0-0" => "1 {$lang->week}",
//"14-0-0" => "2 {$lang->weeks}",
//"21-0-0" => "3 {$lang->weeks}",
//"0-1-0" => "1 {$lang->month}",
//"0-2-0" => "2 {$lang->months}",
//"0-3-0" => "3 {$lang->months}",
//"0-4-0" => "4 {$lang->months}",
//"0-5-0" => "5 {$lang->months}",
//"0-6-0" => "6 {$lang->months}",
//"0-0-1" => "1 {$lang->year}",
//"0-0-2" => "2 {$lang->years}"
);
the permanent ban option is a couple lines below where it shows "---"