Delete Flow Permission for User
DELETE {{base-path}}/sites/{{site-id}}/flows/:flow-id/permissions/users/:user-id/:capability-name/:capability-mode
https://help.tableau.com/current/api/restapi/en-us/REST/restapirefflow.htm#deleteflowpermission
Deletes the specified permission from the specified flow for a group or user.
capability-name
The capability to remove the permission for. Valid capabilities for a flow are ChangeHierarchy, ChangePermissions,Delete, Execute, ExportXml (Download), Read (view), and Write.
capability-mode
Allow to remove the allow permission, or Deny to remove the deny permission. This value is case sensitive.
Permissions
Tableau Server users who are not server administrators or site administrators can delete permissions from a flow only if they have ChangePermissions permission for flow (either explicitly or implicitly).