diff --git a/synapse/api/events/utils.py b/synapse/api/events/utils.py
new file mode 100644
index 000000000..dfefb2662
--- /dev/null
+++ b/synapse/api/events/utils.py
@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .room import RoomMemberEvent
+
+def prune_event(event):
+    """ Prunes the given event of all keys we don't know about or think could
+    potentially be dodgy.
+
+    This is used when we "delete" an event. We want to remove all fields that
+    the user has specified, but we do want to keep necessary information like
+    type, state_key etc.
+    """
+
+    # Remove all extraneous fields.
+    event.unrecognized_keys = {}
+
+    if event.type == RoomMemberEvent.TYPE:
+        new_content = {
+            "membership": event.content["membership"]
+        }
+    else:
+        new_content = {}
+
+    event.content = new_content
+
+    return event