I've tried both of the below and category 18 is still being outputted in the category list.
Version 1
{exp:channel:categories category_group="1" category="not 18"}
<a href="{path='shop/index'}">{category_name}</a>
{/exp:channel:categories}
Version 2
{exp:channel:categories channel="{product_channel}" category_group="1" category="not 18"}
<a href="{path='shop/index'}">{category_name}</a>
{/exp:channel:categories}
Strange thing is it was working previously.