Index: pylib/mozreview/mozreview/errors.py
===================================================================
--- pylib/mozreview/mozreview/errors.py
+++ pylib/mozreview/mozreview/errors.py
@@ -97,5 +97,10 @@
 
 REVIEW_REQUEST_UPDATE_NOT_ALLOWED = WebAPIError(
     1009,
     "Updating review request not allowed at this time.",
     http_status=405)  # 405 Method not Allowed
+
+USE_PHABRICATOR = WebAPIError(
+    1010,
+    "New review requests are no longer supported.",
+    http_status=405)
Index: pylib/mozreview/mozreview/resources/batch_review_request.py
===================================================================
--- pylib/mozreview/mozreview/resources/batch_review_request.py
+++ pylib/mozreview/mozreview/resources/batch_review_request.py
@@ -51,10 +51,11 @@
     WebAPIResource,
 )
 
 from mozreview.errors import (
     REVIEW_REQUEST_UPDATE_NOT_ALLOWED,
+    USE_PHABRICATOR,
 )
 from mozreview.extra_data import (
     AUTHOR_KEY,
     BASE_COMMIT_KEY,
     COMMITS_KEY,
@@ -96,10 +97,21 @@
 You must reopen the review request before it can be updated.
 Review requests should only be reopened if your changes have not landed or have
 been backed out - file new bugs for follow-up work.
 '''.strip()
 
+USE_PHABRICATOR_MESSAGE = '''
+
+****************************************************
+MozReview Is No Longer Accepting New Review Requests
+****************************************************
+
+Please use Phabricator for new review requests.
+See https://moz-conduit.readthedocs.io/ for the
+Phabricator installation and user guide.
+'''.rstrip()
+
 
 class DiffProcessingException(Exception):
     pass
 
 
@@ -396,10 +408,22 @@
                         SUBMITTED_OR_DISCARDED_ERROR))
 
             logger.info('using squashed review request %d' % squashed_rr.id)
 
         except ReviewRequest.DoesNotExist:
+            prevent_new_requests = False
+            try:
+                with open('/mozreview-settings.json', 'r') as f:
+                    settings = json.load(f)
+                prevent_new_requests = settings['prevent_new']
+            except (IOError, KeyError):
+                pass
+            if prevent_new_requests:
+                raise SubmissionException(
+                    USE_PHABRICATOR.with_message(USE_PHABRICATOR_MESSAGE)
+                )
+
             squashed_rr = ReviewRequest.objects.create(
                     user=user, repository=repo, commit_id=identifier,
                     local_site=local_site)
 
             squashed_commit_data = fetch_commit_data(squashed_rr)