Exclude categories from the_category

I’ve been trying to use the code below to prevent certain categories from displaying when viewing a post entry. The only issue I have is that the separator (‘, ‘) doesn’t work. Any suggestions appreciated. The code in functions.php is:

function exclude_post_categories($excl='', $spacer=' '){
   $categories = get_the_category($post->ID);
      if(!empty($categories)){
      	$exclude=$excl;
      	$exclude = explode(",", $exclude);
		$thecount = count(get_the_category()) - count($exclude);
      	foreach ($categories as $cat) {
      		$html = '';
      		if(!in_array($cat->cat_ID, $exclude)) {
				$html .= '<a href="' . get_category_link($cat->cat_ID) . '" ';
				$html .= 'title="' . $cat->cat_name . '">' . $cat->cat_name . '</a>';
				if($thecount>1){
					$html .= $spacer;
				}
			$thecount--;
      		echo $html;
      		}
	      }
      }
}

The code I use to replace the_category in template files is:

<?php exclude_post_categories('2,6',', '); ?>

An alternative solution I found works but for the fact it excludes child categories when a parent category is excluded. My limited knowledge of PHP is preventing me finding a solution.

Any suggestions appreciated.