Loop through entries that share current entry category and restrict to Cat Group Id
You can make use of 'not' to exclude items from your parameters. An example of how this would be implemented in the above use case would be as follows

{% set categories = craft.categories.slug( 'not preferential' ) %}
{% set entries = craft.entries.relatedTo( categories ) %}


If you need to specify category groups or entry sections then use

{% set categories = craft.categories.group( 'groupHandle' ).slug( 'not preferential' ) %}
{% set entries = craft.entries.section( 'sectionHandle' ).relatedTo( categories ) %}