Geeklog does come with an admin tool for comments now. So if you upgrade to the latest version (v2.2.0) you will be able to sort comments by the date and then multi select to delete them.
You can check out the feature on the Demo site. Log in as the admin and then look for the Comments under Admins Only Block. https://www.softaculous.com/softaculous/demos/Geeklog
If you are worried about comment spam in the future you can turn on comment submissions which means the admin will have to approve the comment before it goes live.
One of the Geeklog Core Developers.