diff --git a/lib/client.js b/lib/client.js
index ad74f58af..bc377f130 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -49,6 +49,12 @@ try {
 
 var OLM_ALGORITHM = "m.olm.v1.curve25519-aes-sha2";
 
+var DeviceVerification = {
+    VERIFIED: 1,
+    UNVERIFIED: 0,
+    BLOCKED: -1,
+};
+
 /**
  * Construct a Matrix Client. Only directly construct this if you want to use
  * custom modules. Normally, {@link createClient} should be used
@@ -133,7 +139,7 @@ function MatrixClient(opts) {
         var deviceInfo = {
             keys: this.deviceKeys.keys,
             algorithms: this.deviceKeys.algorithms,
-            verified: true,
+            verified: DeviceVerification.VERIFIED,
         };
         var myDevices = this.sessionStore.getEndToEndDevicesForUser(
             opts.userId
@@ -378,7 +384,7 @@ function _doKeyUpload(client) {
   * @typedef {Object} DeviceInfo
   * @property {string[]} list of algorithms supported by this device
   * @property {Object} keys a map from <key type>:<id> -> key
-  * @property {boolean} verified true if the device has been verified by the user
+  * @property {DeviceVerification} whether the device has been verified by the user
   */
 
 /**
@@ -488,7 +494,7 @@ function _updateStoredDeviceKeysForUser(userId, userStore, userResult) {
             }
         } else {
             userStore[deviceId] = deviceStore = {
-                verified: false,
+                verified: DeviceVerification.UNVERIFIED
             };
         }
 
@@ -508,7 +514,8 @@ function _updateStoredDeviceKeysForUser(userId, userStore, userResult) {
  *
  * @param {string} userId the user to list keys for.
  *
- * @return {object[]} list of devices with "id", "verified", and "key" parameters.
+ * @return {object[]} list of devices with "id", "verified", "blocked", and
+ *    "key" parameters.
  */
 MatrixClient.prototype.listDeviceKeys = function(userId) {
     if (!this.sessionStore) {
@@ -532,8 +539,9 @@ MatrixClient.prototype.listDeviceKeys = function(userId) {
             if (ed25519Key) {
                 result.push({
                     id: deviceId,
-                    verified: device.verified,
                     key: ed25519Key,
+                    verified: device.verified == DeviceVerification.VERIFIED,
+                    blocked: device.verified == DeviceVerification.BLOCKED,
                 });
             }
         }
@@ -550,31 +558,67 @@ MatrixClient.prototype.listDeviceKeys = function(userId) {
  * @param {boolean=} verified whether to mark the device as verified. defaults
  *   to 'true'.
  *
- * @fires module:client~event:MatrixClient"deviceVerified"
+ * @fires module:client~event:MatrixClient"deviceVerificationChanged"
  */
 MatrixClient.prototype.setDeviceVerified = function(userId, deviceId, verified) {
     if (verified === undefined) {
         verified = true;
     }
+    _setDeviceVerification(this, userId, deviceId, verified, null);
+};
 
-    if (!this.sessionStore) {
+
+/**
+ * Mark the given device as blocked/unblocked
+ *
+ * @param {string} userId owner of the device
+ * @param {string} deviceId unique identifier for the device
+ *
+ * @param {boolean=} blocked whether to mark the device as blocked. defaults
+ *   to 'true'.
+ *
+ * @fires module:client~event:MatrixClient"deviceVerificationChanged"
+ */
+MatrixClient.prototype.setDeviceBlocked = function(userId, deviceId, blocked) {
+    if (blocked === undefined) {
+        blocked = true;
+    }
+    _setDeviceVerification(this, userId, deviceId, null, blocked);
+};
+
+function _setDeviceVerification(client, userId, deviceId, verified, blocked) {
+    if (!client.sessionStore) {
         throw new Error("End-to-End encryption disabled");
     }
 
-    var devices = this.sessionStore.getEndToEndDevicesForUser(userId);
+    var devices = client.sessionStore.getEndToEndDevicesForUser(userId);
     if (!devices || !devices[deviceId]) {
         throw new Error("Unknown device " + userId + ":" + deviceId);
     }
 
     var dev = devices[deviceId];
-    if (dev.verified === verified) {
+    var verificationStatus = dev.verified;
+
+    if (verified) {
+        verificationStatus = DeviceVerification.VERIFIED;
+    } else if (verified !== null && verificationStatus == DeviceVerification.VERIFIED) {
+        verificationStatus = DeviceVerification.UNVERIFIED;
+    }
+
+    if (blocked) {
+        verificationStatus = DeviceVerification.BLOCKED;
+    } else if (blocked !== null && verificationStatus == DeviceVerification.BLOCKED) {
+        verificationStatus = DeviceVerification.UNVERIFIED;
+    }
+
+    if (dev.verified === verificationStatus) {
         return;
     }
-    dev.verified = verified;
-    this.sessionStore.storeEndToEndDevicesForUser(userId, devices);
+    dev.verified = verificationStatus;
+    client.sessionStore.storeEndToEndDevicesForUser(userId, devices);
 
-    this.emit("deviceVerified", userId, deviceId, dev);
-};
+    client.emit("deviceVerificationChanged", userId, deviceId, dev);
+}
 
 
 /**
@@ -623,7 +667,7 @@ MatrixClient.prototype.isEventSenderVerified = function(event) {
             }
             var deviceKey = device.keys[keyId];
             if (deviceKey == sender_key) {
-                return Boolean(device.verified);
+                return device.verified == DeviceVerification.VERIFIED;
             }
         }
     }
@@ -1223,10 +1267,14 @@ function _encryptMessage(client, roomId, e2eRoomInfo, eventType, content) {
             var devices = client.sessionStore.getEndToEndDevicesForUser(userId);
             for (var deviceId in devices) {
                 if (devices.hasOwnProperty(deviceId)) {
-                    var keys = devices[deviceId];
-                    for (var keyId in keys.keys) {
+                    var dev = devices[deviceId];
+                    if (dev.verified === DeviceVerification.BLOCKED) {
+                        continue;
+                    }
+
+                    for (var keyId in dev.keys) {
                         if (keyId.indexOf("curve25519:") === 0) {
-                            participantKeys.push(keys.keys[keyId]);
+                            participantKeys.push(dev.keys[keyId]);
                         }
                     }
                 }
@@ -3773,10 +3821,11 @@ module.exports.CRYPTO_ENABLED = CRYPTO_ENABLED;
  */
 
  /**
- * Fires when a device is marked as verified by {@link
- * module:client~MatrixClient#setDeviceVerified|MatrixClient.setDeviceVerified}.
+ * Fires when a device is marked as verified/unverified/blocked/unblocked by
+ * {@link module:client~MatrixClient#setDeviceVerified|MatrixClient.setDeviceVerified} or
+ * {@link module:client~MatrixClient#setDeviceBlocked|MatrixClient.setDeviceBlocked}.
  *
- * @event module:client~MatrixClient#"deviceVerified"
+ * @event module:client~MatrixClient#"deviceVerificationChanged"
  * @param {string} userId the owner of the verified device
  * @param {module:client~DeviceInfo} device information about the verified device
  */