The Wayback Machine - https://web.archive.org/web/20210514084133/https://docs.github.com/en/organizations/managing-peoples-access-to-your-organization-with-roles/removing-a-billing-manager-from-your-organization

Removing a billing manager from your organization

If a person with the billing manager role no longer needs to view or change your organization's billing information, you can remove their access to the organization.

Note: Only organization members with the owner or billing manager role can access or change billing settings for your organization.

In the top right corner of GitHub, click your profile photo, then click Your organizations.
Click Settings next to the organization.
In your organization's Settings sidebar, click Billing & plans.
In the "Billing managers" list under "Billing management", next to the name of the person you want to remove, click Remove.