Посмотрите на следующие опции:
Опция 1: Только что добавленный DISTINCT в SQL запросе
<select class="form-control" id="space" name="space">
<option value="--Select--">--Select--</option>
<?php
$select=mysqli_query($con,"select DISTINCT `Space` from clients");
while($menuz=mysqli_fetch_array($select)){
$filled =$menuz['Space'];
$valuez = array("C101","C102","C103","C104","C105","C106","C107","C108","W1","W2","W3","W4","W5","W6","W7","W8","W9","W10","W11","W12","F1","F2","F3","F4","F5","F6","F7","F8","F9","F10");
foreach($valuez as $value){
if($value != $filled){
?>
<option value="<?php echo $value;?>">
<?php echo $value; ?>
</option>
<?php
}
}
}
?>
</select>
ИЛИ
Опция 2: Игнорирование существующего набора значений из SQL запрос только с использованием "NOT IN". Здесь вы можете использовать подготовленный оператор, чтобы избежать SQL инъекций. Только для демонстрации, я показал здесь запрос со встроенными входными параметрами.
<?php
$valuez = array("C101","C102","C103","C104","C105","C106","C107","C108","W1","W2","W3","W4","W5","W6","W7","W8","W9","W10","W11","W12","F1","F2","F3","F4","F5","F6","F7","F8","F9","F10");
?>
<select class="form-control" id="space" name="space">
<option value="--Select--">--Select--</option>
<?php
$select=mysqli_query($con,"SELECT DISTINCT `Space` FROM `clients` WHERE `Space` NOT IN ('". implode("', '", $valuez) ."')");
while($menuz=mysqli_fetch_array($select)){
$filled =$menuz['Space'];
?>
<option value="<?php echo $filled;?>">
<?php echo $filled; ?>
</option>
<?php
}
?>
</select>