Yep. My perpetual API-wrangling advice. Do the acting in the admin panel and watch the network calls. In this case, here’s what I saw when I chose ‘remove complimentary subscription’ from a user:
endpoint: https://cathys-second-demo-site.ghost.io/ghost/api/admin/members/64bd3bf3f178b200013ea5f4/
PUT
payload:
{"members":[{"id":"64bd3bf3f178b200013ea5f4","email":"catsarisky+testportal@gmail.com","tiers":[]}]}
So it looks like you need to pass an empty array for tiers to wipe out the comped tier. (If the user is somehow in two tiers, you’d want to remove the comped one from the list. This can happen when a user upgrades/downgrades before a plan period ends.)