Only the Fathom Account Owner can cancel the Fathom account.

To cancel the subscription, simply login to your Fathom account.

Proceed to the top right Administration menu (Cog icon) > Account/Billing. Then, click 'Cancel my account' at the bottom of this screen.
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Once you have cancelled your account, all your company data will be deleted from the Fathom system within 30 days. You can resubscribe within this time to retain your company data.

Only subscriptions can be cancelled. If you are on a trial, then simply leave the trial to expire. As you have not entered any payments details, you will not be charged.