Just found this website that says that ‘409 Conflict’ should be used
when ‘the request could not be completed due to a conflict with the current state of the resource’ and ‘where it is expected that the user might be able to resolve the conflict and resubmit the request’, then it gives an example when ‘cascade-delete is not supported’ which kinda resembles my scenario..
http://www.restapitutorial.com/httpstatuscodes.html