<?php

$array['item'] = 'BolleMedRosiner';
$array['item'] = 'BolleUtenRosiner';
$array['item'] = 'Bollemus';

foreach($array as $key => $value) 
{ 
    if (strpos($key, 'bolle') === 0) 
        { 
            unset($_SESSION[$key]); 
        } 
}

print_r($array);

Array
(
    [item] => Bollemus
)