Drupal - Unsupported serialization format on REST/Delete request

In delete method you can not return ResourceResponse. Try return ModifiedResourceResponse.

return new ModifiedResourceResponse(NULL, 204);

Notice: First argument of ModifiedResourceResponse need are NULL, if your specific data for this parameter throw same error.

Also in DELETE method if you want to return a response you could use return new JsonResponse('Processed'); including the following use statement: use Symfony\Component\HttpFoundation\JsonResponse;