Due to a bug in Commerce where I am unable to have admin make payments with a method which is not accessible to the order's user (https://www.drupal.org/project/commerce/issues/2912996); I thought it would be easy enough to simply allow access to the user and then alter the "payment method" form on the Order Information page (/checkout/[oid]/order_information) to remove that method.
But sadly, having no luck altering that form. I have tried removing all the pieces in the form which are related to this; but they have no impact. I have added an #after_build and #process to the main form and under $form['payment_information']. None of these callbacks are even hit (not sure how that's possible).