Display categories not related to an entry
You can grab all ids and exclude them from your query

{% set excludeIds = craft.categories.group('services').relatedTo(entry).ids() %}
{% for category in craft.categories.where(['!=', 'elements.id', excludeIds]).all() %}


If your relation field is in your entry you can do

{% set categories = craft
    .categories
    .section('services')
    .where(['!=', 'elements.id', entry.category.ids()])
    .all()
%}