Unfortunately you can't remove inherited permissions with the production api (v19.0) or the api-unsupported versions. It is technically possible, but it requires using Workfront own internal API, that does not seem to have any guard rails that the standard API's do (or at least it didn't seem to last time I checked), and is not documented anywhere, and getting any support on using it will probably be iffy at best, and I personally try to stay away from using this API as much as possible. If you still want to pursue this, here is how. (Hopefully I have conveyed my nervousness with this api :D)
Method: POST
URL: https://{domain}.my.workfront.com/internal/share/setAccessRules
Headers: "sessionID": "{your session ID}"
Call type: application/form-data
Body:
"objCode": "TASK",
"objIDs": ["target task id"],
"options": {"removeInheritedAccess": false},
"accessRules": []