in index.php of plugin, around line 229 is this code:
$to_user_id = $item['fk_i_user_id'];
$to_user_name = $item['s_contact_name'];
$to_user_email = $item['s_contact_email'];
bellow it, add following:
// CHECK FOR BLOCK
if(im_check_block($to_user_id, $from_user_email) == 0) {
//osc_add_flash_error_message( __('You cannot message this user. This user has blocked communication with you.', 'instant_messenger'));
header('Location: ' . osc_item_url_from_item($item));
exit;
}
Right now, block check is done when sending messages, but in this case it should be also before user create thread.
Fixed in latest version.